seat foam (buns)

Trimming the foam to make them fit is not unusual at all. Don't be afraid to trim it, but keep in mind you want it to be a rather tight fit when you're done or the covers will be baggy.
67 bucket seats are probably the easiest I've ever done because the back comes off revealing the frame. Ever install a seat cover on a 70-82 Corvette? Holey Moley what a job.
Hardest part of the seat cover installation is attaching the listing wire to the frames. Once thats done, the outer attachment is easy.
Take your time, it'll work out.

George