Go with radials. We put radials on our 44,000 original mile '53 BelAir last summer and it was driving a different car. The radials are about the same height but are fatter. We wanted to stay with wide white sidewalls but they were about four times the cost we paid for a set of plain old Mastercraft (Cooper) steel belted radials.
When turning a corner, I used to have to keep turning the wheel more and more thoughout the turn. Now I can turn the wheel to a certain point and hold it steady while the car turns on course.
Also made it a lot more stable at highway speeds, and it handles much more predictably in wet weather. Not to mention it also rides better!
I don't know if there is any truth to the radial/old wheel theory, but we mounted ours on '79 GM wheels, partly because I felt we needed a
6" rim versus the stock 5".
Obviously, we were using the 5 on 4.75" bolt pattern. You might be able to find some more modern wheels to fit your 6-lug pattern (i.e. '71 pickup).
I really don't know that I put a lot of creedence in the rim damage story. If anything, I would think radial would put less lateral stress on the wheels, as the tire takes a lot of the small movements instead of the entire wheel/tire assembly.
There are a lot of stories and half-truths out there and I don't think I would worry about it unless someone can point you to specific, hard evidence. Remember, just because it is on the Internet does not make it credible.
