I am looking at doing a rear disc break conversion. I found a rear end with disc breaks out of a 1980 TA. Will it bolt up under the Monte? Are the rear ends the same size?

 

will the rearend bolt directly in? no. you didnt say what rear you have now, but ill assume its the 8.5 10 bolt which should be the same rear in the ta,but the ta is a leaf spring car with staggard shocks. not to mention im going to assume that the ta is narrower. you could swap over the disc brake set up from the ta with brackets and calipers though to your rear. there are also lots of companies that make disc brake conversions, and im sure theres lots of articles online on junkyard swaps for a rear disc brake set up. actually here you go.
How to Swap GM Rear Brakes to Discs - Car Craft Magazine

granted its a newer car but the bbody drum brake set up should be similar to yours if not the same.

 

The rear in the Monte is a standard 10 bolt. I wasn't sure on the width of two, if they were different or not. It is only worth it if I can do a straight swap. Other wise it will be better to just buy a new conversion kit.

 

yeah it wont be a straight swap. if its a 10 bolt itll be the same housing more or less the 10 bolt 8.5 but itll have different spring, and shock mounts, and like i said i think itll be narrower(not 100% certain on that). did you upgrade the front brakes? i assume that they are a fairly small diameter, and i personally would want my front rotors to be bigger than the rear. i dont know what kind of factory stuff swaps over on the first gen cars from other models. i do know the 3rd gen brakes in front are 9", but can be swapped for the calipers, rotors, and spindles off of the second gen fbody cars, but to do that swap you need to buy tubular control arms(so id look into that kind of thing unless you had money to burn on a whole 4 wheel disc ssbc kit). thats the only benefit of piecing a setup together out of factory parts. that and pads and rotors can be bought at just about any parts store.