Didn't mean to be confusing.
10 x 2.5" drum is stock for the '68 Bendix equiped disk brake cars. That's fine.
A larger drum adds unsprung weight and more rotating mass but might be worth it if you were repeatedly braking from 120 mph to 60 mph or even 80 to 40mph.

When I was describing temperature ranges this was in reference to lining on the pads and shoes. A stock type lining is fine unless you start doing road courses. Obviously there are different qualities in lining material as well as backing plates so later on you can try different ones if you want.

Anyway, whether or not you change to larger rear brakes, it sounds like the system is not working well and needs to be dealt with. That's my main point. Without a proportioning valve, the back drums should lock up when you hit the pedal hard at low speed on gravel.

Locking up the rear first is not good, especially in the rain. Therefore, you got to get the proportioning valve. If you stay with the stock rear brakes, a stock valve should work fine. think Andy F has a new book on B-body upgrades, and it may be helpful in these matters. I haven't seen it so that's all I can say. There were also some magazine projects involving b-bpdies if I remember correctly. You can check the Where to start reading thread for those.

Drag radials. Well think about a pair of tires and rims that match the front. Then put the drag radials on just when you're going to the drag strip. See how things feel with the changes. Save a major tire and wheel change for later when the tires are worn or getting hard.

Not sure what you're looking for in sway bar info. Biggest variables are diameter and attachment methods. Next is how thoroughly they tested fit on different combos. Finally, you may find some bars are hollow which saves weight.

Andy's book or maybe Tom Condran's old book may have some specific combinations they recommend. Otherwise, the best we can say is you're in the right ballpark with your plan. Then you can get more into tuning and adjusting it, if you want.