Hello Jim,
I see you are new to the Barn. Welcome! I have limited experience with firewall welt installation but wanted to get more info from you. Is the old welting still in place? What is your skill level in doing a job like this?
Go on youtube and look at this video https://www.youtube.com/watch?v=O193mmjPml0
The fellow doing the work is not the best craftsman but he does a good job showing how to install new welting on gas tank. I used close pins as he did you could also use spray on adhesive.