I put an entire 142E tank in my old 122 4 door. I finally got fed up with the eternally leaking goose neck on the 122 tank after 3 or 4 resolders and a couple of junkyard tanks. I had to cut the opening a little wider and I put it in sideways, I think - with the gas filler pointed at the original location. It connected to the filler nozzle with a short section of rubber hose and DIDN'T LEAK! A side advantage was that it also had the proper sender for my 140 rally cluster I had fit into the dash (also from that 142E).

It also had the 142E's rear axle, driveshaft, transmission, engine, radiator, seats, and seatbelts. Oddly enough, however, I never put the D-jet on it - the 122's SU carbs seemed to work just fine and dandy on the B20E.