How did you get on ? ( i know this is an old thread) but I've got a similar situation. I've got a changeover year RS turbo (1989). It's pretty much 90 spec throughout, and my tank is leaking. I'm reluctant to drop the tank and then have to put it back (to be safe), and then order..... there seems to be no other way than do that.... unless someone knows better (which i'm sure they do )