I have a 1993 2.5L 200 HP Coffin-Cowl Merc on my V21 and I love it. The only thing I woulds say is make sure you disconnect the oil injection and go with premix. if you can find one the 2.4L 200hp engine is better than the 2.5L 200HP engine for fuel economy, but both are great engines. I can't help you with the shaft length because I know nothing about brackets. 
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
If God didn't have a purpose for us we wouldn't be here, so
Live simply, Love generously, Care deeply, Speak kindly.
(Leave the rest to God) 
Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.
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