Why would a 2 or 3 core raditor run hot on a bone stock 455 engine(.30 over) in a 71 cutlass

 

Not enough surface area to transfer the correct amount of heat from the coolant to the air.
A 2 core (row) would be the bare minimum for a stock 350 without AC or HD cooling.
A 3 core (row) would be the bare minimum for a stock 455 without AC or HD cooling.
A 4 row would be the best choice for any 455.

 

Ahh...Rob gets it. There is one core with multiple rows.

 

Also double check your timing,wrong timing can cause overheating.

 

Not enough advance timing.
rebuilt engine with too tight piston to cyl wall clearance.
water flow to slow.
water pump impeller wore down. (I've seen the stamp steel impeller brake off or lose fins)
spring in lower hose rust away, hose collapse.

Gene