If it is from an SOHC it's from a CB750, 69-76 style. I think it was modified though, split down the middle and widened with a section added. This would also explain the gas cap being too far forward.
The line of the tank (as in shape/contour, not actual line) right where the early decal stripe (not the big one with the black part in the middle) looks like CB750, if it is, it's been altered.