I guess I was the guinea pig on this one, so I may as well post my results in case anyone else is ever looking for this info.
I purchased and installed a Summit 31x19 aluminum radiator. Dropped right in with no problems. The stock Pontiac upper and lower hoses even fit fine. The lower shroud went on without problems, however the upper shroud will not fit without modification.
pontiyak: This is a restoration project! It did not have a radiator, so I needed to make the purchase regardless. Summit aluminum was the same price as the stock replacement 3 core. 400 is .020 over, zero deck, with a Crane 222/
[email protected] cam and 89cc iron heads. For the same price, I'd rather have the aluminum rad.