Assuming the radiator has the capacity to cool the 421, the reverse flow through the heads on the 55 engine vs. the 421's "normal" flow shouldn't make any difference at the radiator. Besides that, you said the radiator was cold while the engine was hot.
More thoughts on "clearancing" the 4 bolt pump: Bolt the pump into the housing WITHOUT the gasket. Turn the pump by hand. If blades hit the casting, pull the pump and install it again WITH the gasket. If it no longer hits, then the clearance in less than the thickness of the gasket. If it didn't hit WITHOUT the gasket, then the clearance is more than the thickness of the gasket.
Clearance testing is a lot easier with the later pumps that use a divider plate. You can check them with the pump removed from the engine.
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My Pontiac is a '57 GMC with its original 347" Pontiac V8 and dual-range Hydra-Matic.
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