Hi john , i have a pair of c7 heads on a 410 in my garage only 8 bolt exhaust could be drilled for cj pattern or plug rear upper drill tap for gt as the manifolds only use diagonal holes except rear although you probably know this as you helped me when i got into this mess 20 years ago !!!!! have gt exhaust manifolds also, merc script pentroof valve covers, and gt air cleaner base, round vacuum motor on side, uses snorkel.in case anyone else is interested! i have some other stuff i would be willing to sell such as a mallory mech distributor with unilite somebody here wanted!!!