Must not be the original tank. I checked the Vin, the frame is a 78.
As for the seat, no worries. There will be a fender on there some day. That air shock only compresses about an inch and the rear shocks are pretty stiff. Im in the test and tune phase with the bike right now. I'm gonna have to change the shocks, their pretty stiff(I bought the wrong ones). Pretty much like riding a hard tail. In fact even if I slam all my weight down on the seat the shocks dont move.