I want to start by saying I think the OP has found a really cool gem here. This body shell Chevy is the only police car so fantastic that police fleets had them refurbished instead of just replacing them. I really love these cars! Built like a tank, fantastic ride, easy to work on, reliable, and cheap to maintain. This one has been a little neglected, but it wouldn't scare me a bit. One thing I noticed is that the areas under the valve cover that get the most oil flow ( inside the rocker arms, and off the tip of the rocker where the oil flows over the spring ) aren't terrible. That would lead me to suspect that the crank case and oil pan aren't jam packed with sludge like the corners of the heads are. I would not bother with a flush if it were mine. I'd just drive it and enjoy it with reasonably short oil changes (personally I'd go 3,000) with whatever oil pleases you. I also wouldn't worry much about the UOA from the fill that was in it, I'd bet that this old 350 will still live a nice long life. I had a similar experience with a sludged up V-8 RWD car back in the late 90's. A friend of mine had inherited a Chrysler M body from his grand father with a lean burn 2bbl 318 in it. The valve covers were leaking so we replaced the gaskets in my garage, it was significantly worse than yours, probably from his grand fathers daily trips to the doughnut shop for coffee each morning. He left it idle in the parking lot every day for at least an hour year round while he got coffee and doughnuts and [censored]'d with his friends. Oil changes were done every 3,000 by a quick lube probably 2/year as he didn't drive it much outside his daily trips to the doughnut shop. There was at least 1/4-1/2" of gooey and some crusty oil covering everything under the valve covers and 1/8-1/4" of gooey sludge on the valve covers themselves. We cleaned the valve covers and got out what we could from the heads and buttoned it up with new gaskets. My friend drove it for the next two years working two jobs, both of which involved putting a ton of miles on his car, he ran a rural paper route each morning and delivered pizzas every night. About 40,000 miles later it was starting to show signs of leaking valve seals so he asked for my help replacing them. We opened it back up and we're both amazed by how incredibly clean it was inside. It was much better than when we put it back together two years prior, you could even see the cast iron castings of the cylinder heads and the casting numbers. The valve seals were all hard as a rock and most of them were at least cracked or completely missing. We installed new seals and buttoned it up with another set of valve cover gaskets. The new valve seals fixed the oil use and blue cloud on start up. He ran it for another three and a half years before a drunk driver hit him and totaled the car, when it got totaled it had 260,000 miles on the little old 318 that was still running fantastic. All he ever used for oil was VWB 10w30 with a Napa gold filter for the entire 6 ish years he owned it always serviced at 3,000 miles. He put between 20,000-40,000 hard miles per year on that car and even with the neglect and sludge prior to his ownership it was another car and not an engine failure that ended its life. I suspect that you'll have a similar experience with regular care on your LT1. Keep us posted! Good luck!