My bike needs tires pretty much right away. It will be on pavement mostly as it takes me exploring, but there will be easy 15-20% fire roads involved. I'm old have no interest in single track, rocks or nasty hills, so the tires only need to cope with moderate speed and the roads will be mostly well groomed. My first choice is Shinko 705 but I don't know if 120/70r17 front and 130/80r17 will fit without modification, which I'd rather avoid. I rode a fat tired Harley Sprint on a lot of dirt roads and don't recall anything frightening except my lack of skill. But that was back in the 60's and it didn't weigh 400 lbs. It seems that decent street tires like BT45 or AM26 would probably give me better wet weather performance and be able to do what I'm asking on the fire roads. Looking for any experience or advice you can offer, Thanks.

I've run 110/70, 110/80, 120/70 front and 130/70, 130/80, 130/90, 140/70, 140/80, 150/60, 150/70 rear tires on my GSs on the stock 3.0 and 3.5" wheels.  All of the tires I've run have been fitments approved by the tires maker except the 150/60 rear.  I've run bias, bias belted, and radial tires.  No alterations are required to the bike for any of these sizes on stock wheels except the 150/60 and 150/70 which come close to or touch the brake rod and may require bending the rod a bit.

Tire I've run on my 97 and 02 GSs:

http://img.photobucket.com/albums/v443/jcp8832/GS500tirelogs_zpsoxtnjzsi.jpg