Battling overheating new 360 build

I should think running a heater would do nothing but help your situation.
Instead of running that "second bypass" hose, it would be simple to temporarily take it out of the equation, with some pipe plugs. Or you could just cut the hose, and stick a chunk of dowel in the hose ends and secure them with a couple of hose clamps. FWIW, a BB exhaust long nut fits the ID of the heater hose nicely.
Expecting an old 318 rad to cool a newly built performance 360 might be overly optimistic.