64Belair
A significant increase in engine Horsepower = increase in heat.
I would be surprized to learn that the rads would be the same unless this was a mild engine swap.
I had a 170HP - 283 ci in a 1962 Impala and transplanted an L-79 (327-350HP) engine. Original 2-core rad intended for the 283, always ran hot and overheated in bumper to bumper situations. I often turned on the "heater" in the summer, to provide additonal means of cooling the engine. You could see the "temp gauge" move to cooler degrees once the heater was on...but pretty unbearable interior heat for the passenger compartment.
If your V8 transplant has any teeth you might want to think about a rad upgrade, of some sort. Some posts here have discussed overheating causes and fixes. Worth the read!!
Regards,
RCE1962