I would leave the Gears alone and just change the Converter. You say its 100% Street, and the 3.91's will have you buzzing down the freeway ~3300 rpms.
66 Dart GT, 402 11.18:109 Best
63 1/2 Galaxie 500XL 406 4Speed 13.20:103 Best
2000 Ram
2005 Durango Hemi.