Not to be a putz, but if you had talked with the people in the old
days who started putting "car" engines in boats, they would have told
you that automotive manual transmissions were tried and generally found
wanting. Marine transmissions have spring or elastic couplings, similar
to those on auto clutches but bigger, and the gears are oversize.

A torsion damper of some type-either a spring or elastic coupling, a
quillshaft, or a belt or chain with a little stretch-is an absolute
necessity. There's a simple way of measuring the torsion flexion which
can be quickly used to troubleshoot the problems: on either side of the
coupling, a "tone wheel" or regularly spaced magnetic or optical 'mark'
is placed and a pickup-a LED/sensor or a simple solenoid coil (think
electric guitar or Hammond organ)is placed. The phase relationship
between the two signals is monitored and as the engine/load are cycled,
where the two signals "wobbles" is where the flexion is happening.
It's a simple test to rig up. OMC and Mercury have been doing this for
years. I know because I worked in a music store in Wisconsin and they
would buy a certain type of internally amplified guitar pickup for this
purpose.