New mav/comet tanks for 74 and up are available, all you have to do is get longer strap bolts.... Last one I saw was on ebay for 159.00 Look at rockauto's web site they may have one...
There's alot of threads about this. The newer gas tanks have a different vapor thing on top too. Got my tank from rockauto
They only have them listed for 1976 models, so that's what you have to ask for. On rock auto they're $103 + $26 shipping charge. and you can get them at napa for $159, no shipping charge. Napa Part #: STP F30 Rock Auto # F30 They're all the same no matter where you get it. I'm running one on my 75 and I was really satisfied on the quality. Its 6 years old now and it's still holding awesomely.
I used a 1969 mustang tank 19 Gal. vented through the gas cap, and lots of room for exhaust pipes on both sides of the tank.... A little more work but turned out nice...