I believe that well set up rear drums with premium front brakes will do an admirable job for even somewhat aggressive driving. Lots of twisties may tip the scales in the direction of disc brakes in back, but if it also requires new MC, lines valves ebrake cables , larger wheels and tires it is a couple grand commitment. I like mine, but if I had to do it again, I don't think I'd spend all the money it takes to do it right.
Just my opinion
I don't disagree on the capability of the OE brake hardware under "normal" conditions. I did have the rears "self energize" and lock up in a panic situation a couple of summers ago. Ending up sideways on a major local highway made me re-evaluate the drums. It moved the brakes up on the to do list.
i too ended up side ways a few times with the drums.
now with the rear cobra disc, it says you should use a 16 wheel but i am using a 15 on them no problem. if you keep your BS to a 5.5 then you won't have any problems. if you want more backspacing then you will need to play with low profile banjo bolts and machining of the brake line fitting like i am right now. with a 15, you could get a 10 wide rim with 5.5 that will work great.
get the cobra rear bakes homie, you'll thank urself later.
Or.. If vaccum is the issue, can't you run a vaccum canister?
on my 521 stroker i had 7 inches of vacuum and the only time it was iffy was at idle. i had no canister, however i have a new one sitting in the garage waiting for the install of the new motor.