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There is no question that OMC 56 triple is the best around 50 to 75 hp motor. The problem is the cost of the necessary nitro case which makes them perform.
Just by chance I came upon some parts that gave me an idea.
There are lots of low cost 35 to 70 hp 2-3&4 cylinder mercury or mariner parts motors out there. These all share the same midsection and similar gearcases with various gear ratios 2 to 1 , 1:86 and 1:64. The case is better performance wise than a nitro and these cases hold up well and are dirt cheap.
The later models have a large cowl that a 56 will fit under and power trim and the shift rod is mounted on the engine pan independent of the power head . No extra fabrication necessary other than maybe needing to alter the length on the shift rod depending on the engine base plate thickness.
Just to see if it is doable I am going with a 20 inch build. I think I may only need to make up two parts.....a short stub shaft drive shaft extension using an OMC 4 spline drive shaft top with a mercury female crank shaft sleeve bottom. The reason for the 20 inch model is it allows me to use a 15 inch gearcase and I only need to adjust the length of the stub drive shaft.
Not sure exactly if I will use a metal plate on the top of the midsection or a Kevi billet plate. The water tube will need to be custom but the mercury water pump will provide enough water.

How will it perform.....ASSUME!

A couple of people have shown interest is this project.
First the key to success will be the motor base mounting plate and will the 3 cylinder OMC parts fit inside the mercury midsection. The opening on a 52 and 59 cubic inch midsection is much larger than the midsection I am trying to use. But the newer mercury models are more costly and a 15 inch gearcase super rare and very costly.

Starting out with a 402 40 hp mercury short shaft got me the 15 inch gearcase. First problem is noticed trying to disassemble the rest of the motor so I can look at the empty midsection and see what is possible in mounting a base plate assembly. This is a mint 402 that I have had 30 years and I don’t want to ruin it. Soaking everything with WD40 and now looking for a parts long shaft 1990’s mercury motor.

I heard from a fellow from the southern United States. He mentioned that some river runners down there mount different powerheads, shorten midsections shorter than 15 inch. The solution they use to make up the drive shaft and coupler is to....cut the lower units drive shaft above the water pump. They square shape the end of the drive shaft like a 1/2 to 3/4 socket depending on the diameter of the drive shaft. They take the powerhead matching drive shaft and weld a high torque premium socket on the end after measuring and cutting the shaft plus coupler to the correct length. Seems straight forward and doable.

This would also be a lower cost solution for making a short shaft gearcase from a long shaft gearcase even when the driveshaft is similar. Example: I have been looking for a short shaft 40 hp Yamaha gearcase to put on a 30 Yamaha short shaft to take advantage of the prop selection available for the 40 hp compared to the 30 hp. With this solution way less costly.