Paul
I dont think the /5 or /6 tanks will fit due to the cutout required on the right side of the fram (when viewed from above) where the electrics are on the ST and G/S.
Im sure that those more knowledgeable than me about airheads will be along soon to confirm/deny my thoughts.
Later R100GS, or R100R (Mystic) tanks will fit..... I have just fitted a Mystic tank to my ST for more tank range, and I think it looks good.
Bubb