You're going with a +3 swinger for woods? You guys must run some wide open woods, where I ride I occasionally have trouble with stock length, and if it wasn't for being able to turn sharp(short track kit) I think I'd be in some trouble sometimes.
TO answer the gearing though, I run stock gearing with 20'' tires on mine....I don't have a 310 but do have a 38MM carb, sectional pipe, and ported cyl. It lugs real good and haven't had trouble pulling up hills yet. I plan on switching to 22's soon, might add a couple of teeth to my rear to offset the change.
Trikes:
1973 Honda ATC70-orange!
(2)1985 ATC70
1985 Big Red 250
1985 250R