I just took a small flat piece of metal and drilled two holes in the plate that matched the petcock hole locations, and made a matching rubber strip from some old rubber roofing.  I then bolted the rubber and plate onto the tank bottom.

When it came time to remove the rust from the top of the tank I used Duct Tape to seal the filler neck, then I used another piece of rubber roofing placed on top of the Duct Tape, then put a flat block of aluminum over that, and then used a bar clamp through the speedometer hole to tighten the aluminum plate down on to the filler neck.  I then placed the tank upside down on a plastic 5 gallon bucket just in case a leak occurred......which it didn't.  It worked great.

Thanks guys