Yes you can trim the foam. I've done this many times in the past. An electric carving knife is best for hacking the foam back. You'll never get a neat cut unless you have a huge vertical bandsaw. To tidy up cuts and smooth things over use and orbital sander or belt sander.
I'd definitely staple the seat cover back on afterwards too. Put a few staples all around until it's lined up nicely, work down the front side to side until your past the angle in the seat, then work from the rear of the seat forwards going side to side again.
By side to side I mean, a couple of staples in the right and then a couple in the left and so on.
I'd definitely staple the seat cover back on afterwards too. Put a few staples all around until it's lined up nicely, work down the front side to side until your past the angle in the seat, then work from the rear of the seat forwards going side to side again.
By side to side I mean, a couple of staples in the right and then a couple in the left and so on.