Most of the aftermarket housings have a huge bypass hole in them.
More bypass = less water through the radiator.

I’m not going to get into a debate about it, but this is “what I would do” to help determine how much of the problem is coolant flow, how much is air flow/radiator capacity.

-get a milodon high flow water pump
-find a stock iron pump housing and plug the bypass hole
-run the milodon pump in the stock housing with no bypass, no heater hoses(heater hose fittings plugged, not “looped”)and no thermostat

If it still runs hot, it’s not a coolant flow problem.

FWIW, I don’t think you’ll have success until you open up the rad support and get a 26” aluminum radiator, and run with a decent fan and shroud.