yes and no yes in that it can be made to fit no in that a Chevy transmission has a different bolt pattern than Oldsmobile's, Buick's, and Pontiac's. the motor mounts are also probably going to be different. However you can change the motor mounts and there are companies that make adapters to use a Chevy transmission with a Oldsmobile, Buick, or Pontiac engine. I have seen an adapter to use a ford transmission with a Chevy engine.
5 QTS.
The 86 2.8L automatic transmission will not bolt up to a 71 Chevy engine.
yes
approx. 500 to 550 lbs.
yes
yes
Pass side, back of motor.
That all depended on what engine and rear gearing you had.
What engine? V8? 18436572 for chevy 1991 silverado v8 350 diagram
Dextron 2 or 3 will work
If it's totally empty you will need between 12 and 13 qts.