Battling overheating new 360 build

30* is plenty,if it's circulating.
and no, you don't have to eliminate the heater core. When you call for heat, the heat exchanger will drop the temp in that line 20 to 30 degrees, about the same as the rad would.
When I leave the heater in circuit, I heavily restrict the primary bypass. I shoved something in there, I don't remember what, with a big enough OD to plug the hose and stay put.And it had a smaller hole in it. The heater core becomes my bypass at the ends of the driving season. During the summer when my feet begin to get hot, I just clamp the hot hose. Up here there's no telling what the weather will be, so some mornings I might still need the defroster in June and again in September, so It takes about a minute to unclamp the line.
Your water pump is a pusher pump. But it's lousy at sucking water up. The only reason it works at all is because generally,lol, you keep the liquid level in the rad high enough that "gravity" keeps water at the inlet.When you put an anti-cav plate on it, it becomes a pretty good pusher.

If your A-body doesn't have A/C, the engine won't care about pulley size, I have proven that multiple times. In fact, my combo is underdriven because I like the insurance at 7000+ rpm that the 14 year old belt is gonna stay on there. I have a manual trans so the rpm is always "up there" until I hit overdrive.
I run a 7# cap.... because if the the engine was theoretically to begin overheating; Firstly, at 7psi the blow-off will be early, and the steam coming out from the hood would be my signal to pull over. And Secondly my 1968 factory gauge is not gonna tell me anything with a higher pressure cap until it's too late, cuz it's is so awful darn slow.In fact, I have so much faith in my cooling system that I never almost never even look at it. Just like the factory oil-pressure gauge. Those gauges are just there, to placate the first-time buyers. The only one that interests me at all is the gas-gauge, the rest might just as well not be there. An oil-level gauge; now that could be useful,lol.