the G3 will probably fit, it will just come up at an angle instead of being horizontal. you may be able to mount the rear support on the seat post, or on the gas tank. biggest problem will be replacing the skin on the inside of your leg, as it will burn off regularly. if you can cut and re-weld the header so that the pipe stays horizontal, you'll most likely have a pedal clearance issue. i'm also not sure it'll run right with a stock cylinder (unless you're porting the hell out of it, and decking the head...). the G3 is really designed for kits, and that's where it shines. i would try the simonini, if it were me.
i made the same carb swap on my seven (15.15 del sha clone), with the same results. i didn't make a custom 15mm ID intake yet, though. have you done that? if not, the carb size won't make much of a difference, as it'll be restricted at the intake. it came with a 58 jet, and bogged constantly. i'm currently running a 68 jet, and it's four stroking at 7/8 throttle. it runs great with the airbox and side covers off, though. i modified the airbox by drilling it full of holes, and i'm downjetting a little (trying a 66 first) to solve the problem. i don't think i'll see an increase in speed until i make a larger intake, though, at which point i'll need to re-tune all over....