Did mine from the wheel well. Just couldn't figure out how to get them in from the cabin/trunk side easily. Even from below it was a pain and I think the passenger side took a lot of cursing to get in and out. They kind of popped in place if I remember right then you could kind of move them around where they needed to be.
I think you are going about it the correct way, just keep trimming a bit until they just barely snap in. That should give you a nice tight fit.
Just in case you haven't seen this thread:
The Novice's guide to mini-tubbing a 68-69 Camaro