My 79 1/2 MGB had the manual with electric over drive. So if you modified it to shift over gear in all instead of just 4th it made for cruising speeds a little nicer. Reverse in all MGB were a challenge to get to go into gear. The only solution was a tear down and polish the counter shaft gears or to replace them altogether with PG straight cut gears. That was in my other 305 V-8 MGB had that kool sounding gear drive whine.
Now my 1960 MGA had a Blown 391 Chevy with a Rock Crusher 4 speed, 6.14:1 spooled gears in a 9 inch Ford housing and a Hurst V-gate shifter. You could NEVER miss a shift but reverse was an interesting task. Although I could only go about 80mph before red lining the engine somehow hazing the tires at 50MPH would scare the crap out of anyone challenging me on the street. LOL
A bit of history on this:
The OD circuit from 68-80 consists of three parts-the dash switch, the OD lock-out switch on the transmission, and the OD solenoid. 62-67(3 sync) is the same except there's also a vacuum switch incorporated into the OD circuit.
On 68-76 transmissions, the OD lockout switch was "on" in 3rd and 4th gear both, so OD worked in 3rd and 4th. This was true of both the "black label" top-fill 1280tpm speedo transmissions and 74.5-76 blue label side fill 1000tpm speedo transmissions. In 77, the box was modified to only engage the OD lockout switch in 4th gear, so on these late boxes OD only works in 4th.
3 OD is a surprisingly useful gear. It's a bit under 1:1, so gives a tiny bit more grunt than 4th but not really a noticeable amount. The big advantage is on a windy road going 50-60mph(where these cars are really at home, even though they'll do 80 all day with or without OD) the 3-4 shift is common. If your OD switch is on the column like mine, you can cruise in 3 OD comfortably, then drop back into straight 3 with just a flick of your fingers and without even taking your hands off the wheel to pull out of a curve or whatever, then just click it back on. No clutch, no moving your hand off the wheel, it just happens.
My car has a blue label box out of a 1980. I didn't install it myself and I actually chronicled a few issues I had on here with the shop on getting the car. When I really started leaning on them after it had been there for 8 months, they had to throw together an OD harness in a hurry(never mind that I'd bought one from Moss and supplied it, but that's one of many things I just have to forget about...) and didn't wire in the lockout switch. So, for now, I have OD in all gears. I have to be careful because OD in reverse is bad news(cone clutch shedding bad news possibly even in a few hundred feet of operation) and it's supposedly hard on the OD in 1st and 2nd although not fatal like reverse. I need to crawl under there and hook up the lockout switch, but also don't want to lose 3 OD .
One of the OD masters up in Pennsylvania can convert 4-only boxes to 3-4, and I'm really regretting not having him do that(and give it a general overhaul before installation even though it works fine).