will a SB radiator work with RB?

It's a HP dependent thing. I'm not sure how far you can go myself. However, my buddy also here at FABO, "The Legend" has a 500 Indy headed ride with a super monster roller in a all steel dart running real well and he has a stock /6 rad in the car now with no issues.

If the rad is clear and flows well, you should be good. From what I have seen.