About to buy a torque converter which would work the best?

I would look in to a 10" if you need more then 2500 rpms stall. Your best bet is too call a few converter companies like PTC and get a recomendation.

I've been running a 10" behind my 340. It works pretty good but was alittle loose with the 3.23 gear. Not very good throttle response around town but it would pull pretty good when you get on it.

I Just switched from the 727 and 10" to a 904 with a low first gear and a tight 8" converter. Between that and the 3.91s it should move pretty good.