Since you have the airgap and a 650 carb I would say 3.91 would be ideal. Maybe even 3.73's would be better if that tire is 26" tall.

But with changes to the cam and heads I might leave the gear alone for now. Especially if you don't mind it and if it happens to dead hook at the track. You might try going to a taller tire after these wear out, (if it will fit) if you are inclined to gear it taller after the changes.

If you keep that gear I might try a Mopar M-1 open plenum manifold and a 750.