1968 Barracuda Notch - Plano, TX
I estimate it is close to 32 gallons.
Tim Grainger had one made for his car, I talked to Hector and made a few changes for mine. Tim's tank comes up into the trunk 2" and is 29" wide. His tank is roughly 28 gallons. His is shown starting on post #66:
http://www.forabodiesonly.com/mopar/showthread.php?t=112873
I had them use the same profile under the floor, but changed two key things. I had it come up 4" into the trunk so it sits about level with the floor where it comes up over the rear axle, and I had them narrow it to 24". I plan to make a raised floor 4" tall that will be flat across the trunk from side to side front to rear. Under the floor I will run all the electronics and anything else I wish to hide. So, once the floor is in place the only thing visible will be the filler neck. This is why I had the tank come up 4". The width I had narrowed to make room for the exhaust between the frame and the tank. Between the frame rails in the back the space is 34" wide. My springs mount into the frame. So the only place left to run exhaust is between the tank and the frame if I want the exhaust to come out the rear. 29" would have only left me with 2-1/2" clearance on either side of the tank, not enough room for a 3" pipe. 24" gives me 5" between the tank and the frame. Now I have room to tuck the pipes between the frame, springs, and the tank. The pictures below show my mock up of a "Accurate Exhaust" 3" stainless Barracuda exhaust tip between the frame, spring and tank looking from the rear and from the front.
You could leave the tank the same way Tim did it, then narrow it for exhaust clearance, it would still be over 20 gallons. I wanted the extra capacity for long road trips and the possibility I would run E-85. If you run E-85 you will use more fuel, and there are fewer gas stations with E-85 so you have to know where they are and plan accordingly.