Speedwaymotors sells a bolt on disc brake kit for the 7.5 for under $300. I am in the process of installing one of their bolt on kits for my 8.5. I have installed big bearing ford ends with a 10/12 bolt GM pattern. The kit was spot on. It would not work with the big bearing ford ends without some mods because they are a larger diameter than the stock GM ends, so I ended up welding them on. But if everything was stock it would have been a simple bolt on deal.
Now, remember, there is more to adding rear discs than just installing them on the rear. The stock combo or prop. valve has to go. It doesn't have the right bias for the rear discs, it has a residual pressure valve set for the drums, which keeps pressure to them to keep the rear shoes close to the drums so they will react quicker. If you add discs with that built in residual valve the rear calipers will drag and cook the rear brakes. I have researched and found the best way to fix this is to get rid of the stock combo valve and install an aftermarket adjustable one. This will require some new brake lines from the master to tie into your new adjustable prop valve and from the master to a T for the the front brakes. Another thing is the disc/drum master. It doesn't have the fluid volume necessary for disc brakes, so it has to be swapped out for a disc/disc master. I have found that a master from a 79 corvette takes care of this. One last concern would be the stock booster. Some say that a single diaphragm booster will be adequate, some say it wont. I have gone ahead and swapped mine out for a dual diaphragm booster from a later g body which was a bolt on deal. Just remember a proper brake system set up for rear discs requires a bunch more stuff than just throwing on a set of rear discs.