Battling overheating new 360 build

I think you need to humor everyone and hook the heater core back up #1. You aren't only losing cooling from the mini-radiator that is the basic principle of what a heater core is, you're dumping it right back into the engine.

That would have been the easiest thing to try vs throwing all those parts at it.

Also how does it run and drive? Does it feel low on power?

Whats the story on the carb? I havent scoured every detail but is it possibly running lean? Attributing to the problem at cruise.