So the electric fan is on the grill side of rad?That would be the first thing to try,change the electric fan to the engine side of the rad and see if the fan is blocking the rad.The next choice would be a bigger aluminum rad.I think the radiator you have is just too small with a 455.
Another theory that is is comming out recently is that on a rebuild one should "zero deck the pistons."That is deck the block so that the pistons are even with the block and not down the hole.Original pontiac pistons are about.020
down the hole and aftermarket pistons are worse.
The theory is you are heating the top of the clyinder walls and less efficent with a piston
down the hole than at zero deck.Cliff Ruggles is
one of the guys that zero decks all engines he does and he says that with that it is possible to use pump gas (92-93 octane)even with a little higher compression ratio.
Of course I find out this and lots of other good engine rebuilding tips AFTER I rebuilt my 389 in my '66 GTO.
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