The only real difference in the subframes of gen 1 and gen 2 is that the early ones were fastened at the top-front with a standard nut/bolt.
The later ones had a tapered allen-head screw going out from the inside (where the subframe also had tapered receiving holes) and held with nuts on the outside of the frame.
AFAIK - there were never two different
lengths to the subframes discussed.
One of the biggest advantages to the later method (that I could see) was that it made removing the battery MUCH less dramatic.
As disco mentioned - the front tool tray will not work with the later seat.
You will need a tray from a 2nd gen.

'79-'80 on left
'81-'84 on right