If the tach is accurate, then you are over propped by quite a bit. However, you engine might not be putting out its Rated Power, for many reasons. So, a Compression test is in order, Is the spark advance working? Is the initial Timing correct? Is the Throttle Cable adjusted so the carb is...
If the Engine is putting out 300 hp with EFI, swapping to a Carb is not going to give much more. If it is putting out 260hp, then the potential for close to 30 more hp is there.
Edelbrock has Marine Manifolds for the SB Chevy
Holley has several Carbs that will work. My preference is the 650 Spreadbore.
I don't know if the Ignition Advance is provided by the ECMs which are fried, or is Independent. You might have to get a Distributor that has it's own advance.
Never heard of them, but that goes for all V Drives
As for the Prop. Diameter and Pitch are usually stamped somewhere on the Hub. A Prop Rebuilder, would have the Tools needed to measure things like Rake and Cup
Some Diligence has to be used, as mixing some parts will result in an Engine's Demise. Pistons, Rods, Cylinder Heads, and Crankshafts are different between the 18/20hp and the 25hp versions,
If it was an Optimax, using premix, would have destroyed the engine in a few minutes, or even quicker, when the Motor's Oil tank ran dry. The Crankcase only sees Air, and injected oil, not Air, Gas, and Oil like a regular 2 Stroke
"@ 80% of WOT"
What do they turn at WOT? That is the Number that matters. 80% of Throttle Lever Movement is meaningless, as the Throttle Lever is not Linear to Rpm.
Since you have doubts about the Tachometer Accuracy, then that should be the Priority. Verify that it is set correctly, if it is...
