Sunbeam, just a thought before you go to a bunch of work and expense.
I'm guessing you're going with a 4 spd AT swap for the OD gear prolly in hopes of better gas mileage???
If your 1980 has stock axle gearing 2.73 or optional 3.31's an OD gear will be useless. The AT will constantly be hunting and selecting 3rd gear cause 4th(OD) will drop the RPM's too low to maintain power sufficient to move the car. In OD you'll just have your foot farther into the throttle to maintain RPM and speed. Stock 2.73 axle gears are already really high. Adding an OD gear might be OK if you live on the hopefully flat Autobahn but for real world road use you won't gain a thing for the time and money spent on the swap.
"Don't mind me. I'm just here for the alibi"