one stumbling block for the cooling system on pont. v8 engines is water pump efficiency. Upgrades by Pontiac include better waterpumps for 69 and overdriven waterpumps for 1970 when they went to the larger crank pulley and smaller waterpump pulley. There are other mods that can also be done including clearancing the water pump impeller to the divider plate, the 421 mod for better cooling between center cylinders, altering steam holes between the block and heads or adding return lines from the rear of the heads to the water outlet to promote more even cooling.
I am changing to the later overdriven pulley style when my 455 goes in in a few weeks and I believe it will improve my cooling efficiency. ( just my opinion of course)
Neill