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As an owner of a 1979 Montauk, I think installing a 25" engine on the boat is a big mistake, Although raising it up 6" on a jackplate will work, it will not look good and will be awkward in rigging, etc.

If I were you, I would have a Yamaha shop remove the 5" extender on the lower unit, and convert it back to a 20" engine. You may find the drive shaft, shift shaft and water tube are even sleeved 5" for the 25" length, so shortening up the engine should not be any more expensive than the jackplate purchase and extra rigging labor.

A 25" 90HP engine is an oddball situation, which is why you probably got it cheap. Changing it back to a 20 will be advantageous all the way around, and for the time when YOU want to sell it.