i remember reading years ago about someone that had a aluminum 421 welded up then drilled and tapped to mount 392 heads, he was hard to catch too, seems it was a mid 60's rail.
wow I knew there was a hemi Ford but never knew anyone did it for Pontiac!!!! Man if you found a set of those heads today you'd pay an arm, a leg, your left nut maybe even the right one plus your first born....lol