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I can help you since I have the same rig you are talking about, a 1979 Montauk with a later model Mercury ADI (alternator driven ignition), power ported 115 in-line 6.

http://smg.photobucket.com/user/lgolt...6919567226

Although I have mine set back 6" on a Detwiler jackplate, my engine is raised to a height equal to two holes up on the transom. This would be the middle set of holes out of the five hole set that Mercs have.

These in-line 6's have a shorter mid-section length that some of the newer engines, one of Mercs tricks for automatically getting higher top speed than the competition by setting the prop higher in the water.

In my case, the power comes on fast and smooth, with incredible acceleration and fast top end around 50. I would use a Mercury 4-1/4 hub Laser II in 19" pitch. With a light load it will turn 5600-5700 RPM.
At 30 MPH cruise, with engine loafing along, I get 5 MPG.

I also have tried a Stiletto Advantage I 19" pitch, and although it runs and holds well, it is no match for the Laser II. It holds down RPM somewhat, stealing top end performance. I am planning on selling that prop, so if you are interested l let me know. It's in perfect condition.

If you are interested in an aluminum prop, Merc's Black Max/Quicksilve Black Diamond, are remarkably good performing and good holding props. I would try 19" pitch.

You can see other photos of the 1979 Montauk rigged with one of these Mercs in the album.

I think these classic Merc in-lines are a perfect engine for the Montauks. They are 20# lighter than the Etec 90's and unlike the old OMC V-4's, don't use an oil injection VRO tank.

In keeping with the classic package look, I have installed 1979 Merc 115 decals on mine to match the year of the boat.