My Dart Swinger Project!

I got a little redneck rust repair done this morning. That bad spot in the inner fender is taking care of. I took a stripping wheel got it good and clean with some acetone and fiberglass patched it on the outside. I'm gonna let it harden and cure and then bondo the inside and block it smooth.
Question what's the easiest way to plug these holes in the firewall? I tried jbweld but it's too runny in just slid out the back it would stay put. I'm scared to try and cut it out and weld a patch in. But I can't get to the back to block the holes off to have anything to weld too to fill them. Help please lol