6 to 8 cyl
In my factory 1967 to 1976 General motors service manual it actually shows the frame is drilled ahead of 6 cyl mount to move crossmember ahead for 8 cyl install and preceeds to mention the trans crossmember is also drilled for same reason also mentions diff yoke length of 2.5 inchs. I have installed both ways as stated and with 6 cyl placement. Just saying what Gm states. Both work, but large distrib cap rubs cab.