I just finished this project on my 71. I used the Tanks, Inc. universal 16 gallon tank. (I know, only 16 gallon! It'll get me around town and through a burnout or two!) The advantage to this tank is, that it's only 7 or 8" deep! It fit between the frame rails with a little help from my 4.5" angle grinder. I "clearanced" the frame rails about a 1/4" on each side, because the tank is exactly the width of the frame rails. I used the straps that came with the tank and I'm in the middle of fitting the filler neck this week. I like this tank's option to use the pickup or use an in-tank pump setup if you want to go EFI later. The Sender that comes with it is a 0-90ohm, so it works with the factory gauge.
I'll try to remember to post a pic later of it with the bed on. You can't see it at all from the back or sides under the truck.
Good luck with your project!
http://www.tanksinc.com/index.cfm/pa...rod/prd279.htm