I have a 1978 Barth, that I jut found out the gas tank is leaking at the seams. It needs to be replaced, the problem is, I don't know what type of tank to even ask for. Are there different sizes? Was the tank a stock item?

Mine is welded, presumably at Barth. Unless your old tank is rusted out, a leaking seam can be repaired by a decent welder.