You do NOT want anything like an absorbent foam rubber between tank and underbody. My 76 power wagon had absorbent foam rubber between tank and cab. It held moisture against the steel fuel tank and the cab and rusted pin holes beneath the foam rubber.
I suggest using 30# roofing felt between tank and underbody.
I've parted out many dozens of a-bodies years ago. All of them had what looked like roofing felt in there.
