Chances are you would need more than a lower unit to convert to a shorter
shaft. The tower housing (part of the mid section) has to be changed also.

I would install a jack plate that is capable of being set for 2" of setback.
Use the same mounting holes on the transom. Mount the jack plate as high as
possible. Then mount the engine as high as possible. Then adjust so the
cavitation plate is 1" above the bottom of the boat w/ the motor trimmed
flat. The extra stress is not a concern.

More setback will increase top end speed but will cost you on hole shot for
skiing.

Since you will have to change props anyway, I would recommend a good 4 blade
SS prop for extra hole shot performance.
You want your rpms right at max for best hole shot. Try a few props before
you buy. Guess is you will need a 21 or 22 pitch but that is a guess.

Tony
My boats and autos - http://t.thomas.home.mchsi.com



wrote in message
...
Ok I'll give some more info, I have a 16' 4winns Marquis with a 75hp.
Mercury 20" transom on it. The problem is that the 75 is not enough
power for slalom skiing starts so I bought a 120hp. Johnson, the 4winns
is rated @ 125hp. Maximum. Then I found out that the Johnson has the
extra longshaft, after I bought it of course. So now I am trying to
decide what would be the best way to go in terms of cost and
performance. Any advice? I was concerned about the extra stress (if
any) on the transom from lifting the motor an extra 5" on a jack plate,
or any possible performance improvement from the Prop being further back
and away when trimmed up at speed if anyone has done this. I was also
planning on mounting the jack plate on the pre-drilled mercury transom
mounting holes thereby eliminating the need to drill more holes in the
transom. Or would it just be easier/better to find a decent used hull
with a 25" transom. Who made a 25" transom in a 16'-18' ski/fish open
or closed bow V-hull boat? If any one knows. Thanks. Tom.