Here is an answer I got from Gus at Vans a little while back:
> RV-6 with O320, engine mount=WD-762-PC, cowling kit=6 COWL KIT S O-320
> RV-6 with O360, engine mount=WD-762-PC, Cowling kit=6 COWL KIT S O-360
> RV-7 with O320, engine mount=WD-757-PC, cowling kit=7 COWL KIT O-320
> RV-7 with O360, engine mount=WD-762-PC, cowling kit=7 COWL KIT O-360
I think the difference with the 7 is a W&B issue.