Plymouth usually used one strap across the tank connected to the frame rails. Dodge used 2 straps lengthwise.
I have this one from a 65 Valiant wagon. If you're interested in the strap, PM me.
You can use moisture barrier material that would go under siding on a house. I have some that is about a 1/16" thick
and that is what my son used on his car and I consider it very similar to what the factory used.
My son has a 71 Swinger as well. He had the same fight with a new tank, grommet was to small. I can't remember
if he modified the grommet or not. I will talk to him today and get more info. I know he used silicone grease.
This is not related to the fuel issue. I have a Miller plasma cutter that was less than a year old. I hadn't used it for a few months. Went to use it and it didn't work. Took it to the place I purchased it from for repair. It turned out there was an insect nest in the air inlet. Strange things...