Cab mounts causing sag?
Did you try to adjust the latch / striker?
It's before it would even hit the striker
Then your only option would be to bend the hinges up. Are they pulling away from the cab?
Worked at body shop for a few years. Wish I was there to help ya out. What does the body lines look between the fender and door? Is it even? wide at the top? Has it always done this or just start saging?
I wouldn't assume the body mounts are just because the fenders went on fine. Get under there and take a look. I had the same issue and it was the body mounts.
Check the hinge for out of round. Sometimes they become elongated or oval shaped. Could b your problem. GM r known for heavy doors