The only better choice than a TH 350 for your truck would be a TH400. Manual transmission vehicles also need to be equipped with a hand crank for starting and an acetylene generator for lantern style headlights. All those things are the same degree of obsolete. With the right rear end ratio, having an OD gear in the transmission becomes a moot point. Make sure the boat engine is a clockwise rotation type- - - -the CCW engines have a lot of special modifications to the camshaft drive, and a rear main bearing seal that is unique to that type of engine. Wrist pin offset in the pistons might also be different.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!