Of all places, I found this ad on a LS1 FBody site listed by the persons son. Looks to have the 425 motor? -Dan
Only motor offered for the 64' Riv was the 425. Hopefully it wasn't replaced at some time with a 401.
I thought I put the link in the original posting - maybe not... http://www.ls1tech.com/forums/showthread.php?t=847021&highlight=buick -Dan