If it's a fl350, I would be nervous about running that much power through that transmission. My 350 broke the trans with a pure stock motor. Others using sled engines in 350s haven't widened the frame, but had to re arrage some frame tubes on the gas tank side to allow the engine to move over enough to line up the clutches.
32:1 should be fine. That's what I run in my 597 Rotax. I have no suggestions on the water pump, but radiators for a Honda Civic off Ebay have proven up to the task of cooling sled motors.