Keep everything that was there. That way when you put it back together there is a perfect fit.

Just use a bunch of 30 minute epoxy and don't hesite to use a lot of it...

Then add some tri-stock to the rear AND front of it. Usually it's easy to do right after you've put the firewall back into place. That way there is only one job and it's all all wet glue so everything fits.

Some pins through the fuselage sides and into the firewall can really help as well. Use toothpicks or little pieces of plywood. Just drill little holes and stick them in with some glue.