I'm working on a project that involves a 56ci OMC, a 15" mid and a V4 gear case. the engine will most likely be something like 16" because I need an adapter plate between the mid and gear case. The engine will be for my 15' tunnel used for Classic Offshore racing. Well that's the background...
I need to figure out how to best adapt and shorten the driveshaft of the V4 case to fit my -92 engine (multi spline)
My options so far seems to be:

Shorten the existing V4 shaft and have new splines cut. The diameter of the shaft is perfect but without the right connections the price might be high for the machining.

To get an upper part of a 3-cyl shaft and brace it to the V4 shaft. I think this would be quite straight forward and I can do the machining but it requires a 3-cyl shaft...

Square hole sleeve to couple the two shafts, this seems quite simple as well but requires a 3-cyl shaft and a square hole sleeve, My brother can do the machining of the shafts for me which is nice. I think there is room for the coupling just under the engine as well as just above the water pump, what would be best and why and would a 12mm or 1/2" coupling survive???

Please share your thoughts and experiences!

//Per