Running Hot?

sometimes 'mildly built' vs 'heavily modified' is not the differing factor. It's the way the engine was rebuilt that can make all the difference. A poorly rebuilt motor may run inefficiently and therefore run HOT. Just throwing that out there up front. That doesn't mean you have to go get your engine rebuilt again. It just means you may need to add more cooling capacity to compensate.
Depending on the answers to Dana67Dart's questions may determine whether an upgrade is needed and whether a 3 row copper/brass radiator is enough.

You don't want to upgrade your radiator if it is something else but you certainly don't want to purchase a middle of the road upgrade if it means having to do it again because it wasn't quite enough to solve the problem.