Because the new trim is dimensionally narrower and shorter in height than original I had to bevel the perimeter edge to almost a knife edge to fit it. I did so after cleaning up the factor cut outs (I think they might have used a chain saw) by reinforcing the cracked, sloppy corners with fiberglass cloth and epoxy resin. I used a 4" brayer roller to squeeze it into place. It was a mm by mm arm wrestling match! I found that the best procedure was to start with the most difficult corner at the front right bottom and work back to the top of the door opening at center. I had to slowly pre-bend the corners before walking them in. Do NOT use a heat gun.
Sometimes (many times) in renovating these beasts, we have to finish what the factory started...they were built to a price point, and time=money, so, as long as it passed visual inspection...off it went.
My '74 Elite 503 left the factory with paint issues (I got the original build/inspection sheet from Mike Pomfret back when) that ended up having to be 'rectified' once it was off the ship on this side. Guess they figured it may get banged up on the way over, so would have to be cleaned up anyway.