The engine you have definitely was run at 1800 rpm as that is the standard generator run speed for 60 Hertz A/C power here in the USA.
As I see more of what your doing, & I run some numbers over here, you definitely need to go with the 10 degree down angle Twin Disc MG-5050A transmission.
It will handle the engine HP as is now or when you up grade it to higher 365 HP neighborhood as well & it is rated for up to 3300 engine rpm.
For your displacement & design & engine I would think a transmission ratio of 1.8 to 1 ratio in the trans would be good for 12 knots at 2600 rpm with some propeller pitch tweaking depending on what you do with the engine & HP available.
Obviously you may go with the engine as it is now, which if not after cooled is actually about 220 HP & later up grade to the higher HP options you may want, at a later date.
And then you could also run all day - 24 / 7 - at 8 knots running in the range of 1600 to 1800 rpm and get great fuel economy.
Let me know if I can help.
Sounds like a fun boat.
Good luck.
Alfa Mike