Re: Thermostat replacement - 4.3LX
Why don't you get the right parts, then you don't have to worry. I would be less inclined to worry about the raw water pump impeller in a "new to me boat" than the thermostat. Then there is the ujoint bellows an alignment issues. Manifolds and riser condition. All that can kill an engine and/or an outdrive.
Anyway, back to the thermostat, you need a 143? thermostat for the raw water cooled engine along with either 1 or 2 gaskets (depending on which design you have for that year), and the ORing that holds the thermostat in place.
If you can post a SN for your engine, I can give you a link to your manual. Your 94 may be the boat year, but not the engine year, and you are right at the changeover.