You're correct, the '77 Ford had a 22 gallon tank behind the rear axle. The auxiliary tank could be any size. Since you'll have to remove the tank to install a new sending unit in the top, I'd drop the tank, measure the depth of the tank from top to bottom, and get an appropriate unit. It really doesn't matter on the gallon size of the tank, just the height of the tank. When you reinstall the tank, you can take it and fill up and find the capacity.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI