I've taken a look at the parts catalog for the yanmar here...
and it says there are 4 main bearings in a set... you've got to have one set at the front and the rear of the engine, thats 2, then, maybe one set each between cyls 1 and 2, then another between 2, and 3? if thats the case, your crank should be able to handle the load.
It looks as though you have a non-dedicated belt running the alternator from this picture....
If thats the case, I'm not sure a really hi-output alternator is the way to go on a single belt. The crank pulley looks to be about 180 deg. contact area, spreading the load out, but the contact area on the alt. pulley is pretty small.
heres the tech drawings
the optional 80 amp alt. mounts on the opposite side of the motor, that may help a bit, or, another solution may be to add a pulley to your crank, and use that alternative site to add a second alternator, If you've got the space.
The Balmar site has some thoughts on pulleys, belts and alt. sizes here...
Another question to ask about this stub/pillow block installation. If your motor will be installed with isolation mounts, I would think that the normal vibration of the motor would cause wear and stress coming from the hard mounting of a remote pillow block as well, unless you iso mount that too. In addition, unless the stubshaft and mounting are in perfect rotational balance, thats going to add vibration as well.
Please keep in mind, I might be suffering from a cranial-rectal inversion.