Juat tried to do this out of curiousity, and it would be possible if you bade a adaptor plate of sorts.
mount the plate to the existing mount hole, plop on the new ('79) tank, and mark where you need your mount hole.
remove plate, drill new mount hole.
However you will have problems with the stock '81 seat pan interfering with the new tank.
Unless you are willing to replace your seat/modify seatpan and re-cover, this is a bad idea....
and the '79 seatpan mounts WAY differently, unless your willing to modify it too.
l8r
l8r