> are all the gas tanks '80-'91 interchangeable?
> (can i put a '80 gas tank in a '91, or visa versa, or whatever)

They changed in late 1985; the filler neck got smaller. You can interchange
them but must use the correct filler neck and related parts.  FWIW, I have
brand new tanks of either design for $199.95 (including listmember
discount).

- Ron Salmon
  The Bus Depot
  http://www.busdepot.com