I'm going to assume your 48 Chevy has the same fender dimensions as my Olds. If you are planning to lay frame, the biggest tire you will get forced under the rear fender is a 205/65R15 or similar (about 25-1/2" dia.) Even then, that will require notching a 1/2" tub into the body. The front can go a bit bigger. I run 205/75R15 on the front (about 27" dia.) but I think I could go a little bigger still.
I personally don't worry too much about something in the rear dragging if I have a blow out, as long as it won't try to dig in, but anything that hits ground in the front is gonna try to plow and cause steering failure. Those A-arms are bad news I'm afraid. Too bad because they look good.