If they bolt up the gear ratios are going to be different. If you put the 120 on the 70 you will need to down size the prop. If you put the 70 on the 120 it may not be able to handle the power. If you are changing the LU on a 120 almoct any of the 3 jaw reverse Merc units of similar HP should bolt up and be the same gear ratio. I put an older Merc on my 120. Bolted up fine, shifted, etc all the good things. Note the 3 jaw reverse! A 6 jaw unit will not work without changing out the shift mechanism.. Once you replace the LU it may need to have the shift linkage adjusted to get to the sweet spot in neutral so that the clutch does not touch either gear or at least it is minimal.
A POINT. If you haven't dropped the lower unit before, be very careful of how you have the shift lever. The replacement LU must be in the same gear position. Many techs say forward for both, but I had mine in neutral the last time I dropped/reinstalled mine.
