I had a problem like that when building my Ultra Sport.
The firewall ended up slightly askew after the epoxy
had set. Even though I was sure everything was square
when I put the clamps on.

So out came the heat gun. Get the epoxy nice and hot.
It will loosen up. If you get it too hot it will begin to
smoke. I would imagine that would not be good to
breathe so provide for plenty of ventilation.

Mike Hammer