If it was me, I'd give that old radiator a chance. They're pretty darn big and I have NEVER had any type of overheating problem using one, especially in a truck with lots of space under the hood. If it's not plugged up with crud, I don't think you'll have a problem. I'm using a stock type copper/brass in my Mustang with 500hp and it doesn't overheat. That truck has a ton more area. I've driven nothing but early/mid '60s to bumpsides my whole life (the newest truck I've ever owned is my current '70 F350) and I've never had a single problem that was radiator related other than bad tubes leaking water, and then I replaced it with a good used one.