Drivabilty vs. Performance help newbie

Stock 340 converter stalls about 2400 RPM and that's what I used for the 727 behind my own 340 when I had the transmission rebuilt. If your idle speed is anything reasonable that kind of stall shouldn't be an issue with a properly dialed in cam, carburetor, and timing, especially with the 3.91s masking any overcamming issues.
I agree with other posters, optimize what you've got before you start making changes, although I'd be leaning toward using a Thermoquad/Quadrajet type carburetor (that new AVS type Demon is said to fit square flange manifolds) when you start playing around with axle ratios.