A good radiator shop should be able to swap the hose nipples to accommodate the V8 radiator hose sizes. They're just soft soldered into the tanks. However, that's not the main problem you're going to encounter. The core won't be rated to handle the pressure you'll need to run with that engine. That core design is intended for a pressure cap somewhere in the 4-7 PSI range, and your 400 engine is going to need a pressure cap somewhere in the 15 PSI neighborhood. Even if the core doesn't leak now, it won't survive a high pressure cap. Pressure test that one at around 5-7 PSI, and if it's good, sell it and buy a new one that's pressure rated correctly. You'll also need a heater core that's rated for high pressure.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
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