What axle ratio do you have now/anticipate installing?
Divide 10 by that number to get a useful 1st gear suitable for both strong launch and good manners in low speed traffic.
How fast does your cam need to spin to get torque? This is related to your minimum cruising RPM, you can figure this from your tire OD, axle, and OD ratio if any.

The 6L80 looks like a good choice, it has a much stronger 1st gear than the TH400 (will pull a really tall axle ratio), and deeper OD than 4L80, GV etc.
1 4.027:1
2 2.364:1
3 1.532:1
4 1.152:1
5 .852:1
OD .667:1
Spread is huge at 6:1