Welcome here! If it is a custom (frame number starts RC05 like for '80 on standard), I think you will need a '80 on standard model or custom tank. Note that the petcock thread diameter changed from 80 to 81 and the 1982 SC (RC08 frame) had a quite different tank again. Also 1979 standard and European Z-model (RC03 frame) had a longer tank that does not just fit.