Ethanol vs non Ethanol gas octane issues

No Cats on that beast why not buy airplane gas? 100LL. The LL means low lead but it has 5x the lead needed in most small plane engines but Your GM will thrive on it. It’s probably the best for long term storage as well. Most small airports will sell it by the can or at least did. They might even let you drive up to the pumps if you ask them.
I’ve run booze gas exclusively in my 90 Mercruiser V6 4.3 with 2 BBl Mercruiser ( Rochester) since it came out. I’ve likely put 5-600 hrs on it since ethanol came along and it’s now got 1500 total I did change the fuel lines out to be alcohol resistant . Mine sits in a cold barn through all the Northern NewYorkistan winters throughout this period with ethanol gas in the carb bowl as often as not. It’s damp around here like an arctic swamp as I like to refer to it.
My buddy and I tore that carb down 4 years back as the choke plate was stuck fast and it was just time after around 25 years and 1200 hours to put in a kit. No alcohol gas damage or pitting to the carb at all surprisingly. Just the usual dried out gaskets and accelerator’s pump.
The one issue was the rod the choke butterfly . The rod had literally welded itself to the sides at the pivot points. I do mean welded as a gentle vise trip and punches wouldn’t touch the stuff. That was surprising so we just drilled it out and put in a new shaft. I assume it was the fumes rising from what was in the bowl that did it during layup. Anyways back together and it has been as usual since. I was completely surprised to see no signs whatsoever of pitting in the carb body .
So FWIW that’s my Northcountry experience with E10 long term in something roughly similar. Take from it what you will. I’ve seen absolutely no issues with big carbs while my dealings with small engines is totally opposite. Castings pitted, bowls eaten through and I usually replace or tear down at least one a season to magically run fine again. Nowadays if I run booze gas in small engines I put on a shutoff valve in line and use that to shut it off.
My 1970 Chevelle SS 396/350 HP stock motor requires premium fuel. My problem is that I can only get 90 non ethanol gas locally and my car wants higher. I can get 93 ethanol gas but I do not think its better for my carb (newer unit). Currently I am using the 93 ethanol but also adding Starbrite Star-Tron stabilizer to counter the ill effects of the ethanol. My concern is fouling the plugs with all this crap. Opinions

I do not store the car for months on end but obviously don't drive it everyday.
From what I've read gas goes stale well before ethanol falls out of phase, which I assume is what you're worried about.

Why not just confirm whether or not the fuel line and carb are e10 compatible? If not replace them.
100LL may not be an ideal choice for many reasons. Not the least of which is the high lead content that will contaminate the oil, leave deposits and foul plugs.

However, it's Stoicheometric ratio is very close to 15 to 1 and will operate cleanly in many engines from 11 to 1 all the way to 17 to 1. It contains no alcohol of any sort, and evaporates more readily than auto fuels. It is designed to evaporate at high altitudes and very cold temperatures.

I think you are referencing it's Reid Vapor Pressure, 100LL's RVP is greater than 38.

Anyone who has thrown a sumped sample of 100LL on the ramp knows it evaporates very rapidly and completely. Unlike auto fuels which tend to leave a puddle. Avgas is mostly Alkylate with under 7% toluene. Avgas will run very well in his engine at any altitude. It's just not a good choice due to contamination.

That has not been my experience nor the experience of many racers.

Avgas DOES contain forms of alcohol as de-icers such as isopropyl alcohol and ethylene glycol monomethyl ether.

Avgas has no defined stoich air/fuel and can vary wildly by batch and supplier. In most cases I've seen, a carb going from E10 93 to 100 LL wanted more jet. (by 2-4 numbers) They also wanted 3-4 degrees more timing for best power and were still down on power and response compared to E10 93, VP C9, and VP 110. (these were NA applications) Though I've heard of people having to pull jet out with avgas, I have not personally experienced that. In speaking with others, it seems it can vary from as low as 13.1 to as high as 15.2. It doesn't matter much for aviation where you can adjust fuel mixtures at different times in flight, but can be wildly inconsistent for a car engine at sea level.

In distillation, avgas hits 90-100% at a lower temperature, but doesn't start until a higher temperature.

Distillation ------ E10 93 ------ 100 LL

10% ----- 120°F ----- 167°F
50% ----- 180°F ----- 221°F
90% ----- 325°F ----- 275°F
100% ---- 403°F ----- 338°F

A large reason for this is the use of toluene at 15-25%. (per Shell's SDS) That higher 10-50% evaporation will make a car engine more stubborn to start in cold weather on 100 LL such as seen with other higher 10-50% fraction fuels like E85. Those engines mentioned above did not like trying to start in 20°F weather.

Avgas requires an RVP of 5.0-7.0 psi (most 5.0-5.5 psi) where as many E10 auto gas blends are 6.0-10.0 psi depending on climate / region.

The avgas is designed around engines holding moderate, steady rpm for long periods of time where engine response isn't a factor. It's a major factor for a car engine that sees much more frequent stop and go. Driving on it myself, the engines felt sluggish and unresponsive no matter what we did in terms of fuel mixture and timing. When at the track and you lose .05 in the 60 ft and down 1-2 mph at the 1/8th, the engine is not happy. It's a noticeable bog when you hit the throttle.
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Assuming all the rubber parts have been upgraded to ethanol compatible, run the E10 93 octane. As others said, just be sure you run it regularly and also be sure to add fresh gas at least every 4-6 months.
Goodness I'd back off the timing some before I did all that searching. You could add something like an MSD timing control?
My mustang sits all year until I see it for Thanksgiving while visiting my folks in southern Ga.. Fires right up every time on 87 e10 gas since 2016 (use sta-bul in it). My F150 sits for months at a time with either e10 87 or e0 87 and never a problem with either (use mmo in this one). I have found zero difference with using either e10 or e0 or letting them sit for 6 month or 11.5 months.
I use 10% ethanol in everything I own, but I also use TCW-3 @ 1 oz per 5 gallons. I never have a starting issue, even on things like snowblowers and lawn mowers after sitting for months on end.
Automotive fuel systems are sealed anymore for the most part. So there is no way for moisture to get in unlike a vented tank on a small engine.

I have B&S motor that has the carb in the tank on a pressure washer. The fuel pickup does not go the the bottom of the tank. And as time goes on water builds up in the bottom until it is enough to stall the engine. No way to get it out other than take it all apart and then use rags and compressed air to dry out the tank.. No way to dump it to get 100% out.

I put a drain plug in the tank but the metal is so thin it does not seal completely. So this will drain the tank on it's own.....