Go 2 speed, run 10w30 non-syn, upgrade your carb to a 15+ (ire rick makes great intakes), get new brake pads and tires, check your cables, and absolutely replace the exhaust with an expansion chamber.
If you are really 320 lbs you should consider replacing the rear shock (I think that the recommended max is like 160-180 on the NC/NA frames).
If you have any ?'s along the way just post; there are few kitted express around now (finally).
I personally prefer the wind-up/points version the best, but 2 speeds are nice for the big guys.