You sure its the tight gears that causes a stumble ? I have a manual valve body trans and can lug the engine in 2nd or even 3rd gear while going very slow-yes I know that isn't good for the trans
It might be that air gap intake, carburetors need some heat to run well on the street. Yeah if you live in Texas you need no intake heat during the summer, lol but I found up here in Michigan, even 65 degree temps at night, a non heated intake isn't so great at low speeds
The carb might need some fine tuning as well and don't overlook the ignition system.
As for the perfect gear, heck I feel the 3.91 gears I got, still is a compromise. That 340 seems to still want more gear, like a 4.30 to 4.56 for best ET's but not for better street manners, just for quicker runs
A 3.23 gear does make a great expressway gear with no OD