I used 30# roofing felt from the hardware store for the tank pad. It was basically the same stuff.
It doesn't really bother me having 4 gallons less gas. This car is a weekend cruiser. I'm not taking road trips so the extra 40 miles of range I'm giving up doesn't really bother me.
I did use a b body tank in my 68 fury. It was a little narrower but bolted up just fine. The tank was for a 68 coronet. I lost 4 gallons of fuel capacity. I also used the sending unit for the coronet tank.
I purchased and installed a set of these. As mentioned the bolts fit very tight in the slot so there was no chance of them falling out during install. The clip themselves also fit quite snug in the finish panel itself. Overall I am very pleased with these.