You probably had a coronary when you saw the price! You can buy a nice rear disc kit.
8.8 axles are suppose to have the same bolt pattern for the backing plaes as the small beaing 8 & 9 inch housings. I understand that Aerostars used a big rear brake that will bolt on. Also I've seen F250 pick ups with the 8 lug 16" rims using the 8.8 axle.
So what truck are those cobra automotive drums and backing plates from? Surely cobra didn't have them specifically made/cast. Probably much cheaper to order the parts from the local auto parts place??