Hmmm, that's interesting. Not accounting for the torque converter, we essentially have identical final drive ratios. You are 4.0 (1st in 6l80) x 2 (np205) x 5.38 = 43.0 I am 2.5 (1st in th350) x 3.8 (Atlas II) x 4.56 = 43.2.My race car has a 6L80 and a Np205 (1.96:1) and 5:38's. It does good at what it was built for (KOH), but sucks trail riding and technical stuff. It is not slow enough. I would think a 4:1 case would be better but if you have the room I would do a 4spd atlas all day long. I had a Black box-205 combo in my last rig. Never used the black box racing but did enjoy it trail riding on tech stuff.
If you had a 4:1 t-case or a 2:1 doubler, you would be 86.0:1 final drive ratio and with an automatic that seems very very low? I think the rule of thumb is the torque converter provides another 2:1 reduction, so almost a 172:1 effective final drive ratio? My other 40 has an H42/rock box/4spd tcase/4.10 diff drivetrain and that is right at 100:1 in low low. Unless you're building a moon buggy on stickies, I cannot see the utility of something that is geared much lower than 100:1, especially with an automatic?