Assuming the radiator in your truck is like the one in my '54 car, you should have some extra capacity. My '54 is a 3-row all copper/brass model. If your radiator is in good shape, and you've got a really good fan & pump, you should be able to cool a 325 HP 355 (350 bored .030") to about 180F in 90 degree ambient.
Anything bigger might get hot. I've heard that the 400 small block has a tendency to run hot due to its water jacket design. (Anybody know?)
A bigblock might be too heavy w/o serious frame/suspension mods. You'd have to run an external auto tranny cooler, cuz your radiator won't have a built in cooler.
Another concern might be your rad core support. If a V8 won't sit back against the firewall, you might have to move the rad or support forward to clear the motor. Then it'd almost be easier to use an aftermarket performance radiator anyway.
Or you could drop in a '56-62 235, and have more time to get the Pepsi out of the doorlocks.
Good luck,
chip
