If your 4.3 was CPI you might be ok on the FP, if it's TB injection it won't be enough psi.
As for the radiator, you will be going from a iron engine to an aluminum one, this means that now, any overheating will be catastrophic. I would want a good sized aluminum radiator to match...Mike.
fwiw Your old FP had a fuel return system and I believe the 5.3's did away with that so that may make the interface a little tricky.