Chuck, Berco made snowblowers for direct fit on the 300 series, they did not require any modifications that I am aware of.
I am with you that 2 stage is much better suited for coastal new england snow. I had a 2-stage for a Sabre that I debated making fit. I personally would not be snowblowing with a JD if I didn't find a model 47. I also get water run off from the road, so my situation is a little worse then most.
I don't think a conversion would be all that bad, I would design it similar to what cub cadet did for their belt drive snowblowers for hydrostatics, with horizontal shaft engines. very similar to a 300 series mower mule drive. it uses a couple pulleys below the pto to route the belt. you can see two small pulleys in the attached picture of an ariens. it is a vertical shaft engine, so the pulleys are a little further forward then they would be on your 300.
the other option I see, is to use a mower mule drive to send the belt back to where it would normally go on you snowblower, you would have to get creative to get everything hung underneath though
the trick is the pulleys need to be in a stationary position in relation to the pto, but also at least one of them will serve as a tensioner, so it needs a spring to pull it vertically downward. hinge the blower right in front of that, and use a push rod from the rockshaft on you tractor to push the blower to the up position.
the other option is to use a mower mule drive to direct the belt back, to the standard drive pulley on your snowblower, but you might run into clearance issues.