as you reduce the final drive ratio, remember the engine only has so much torque available. If your running out of power with 4.11's then you will run out of power that much more quickly with 3.55's Just because you can pull 2700 RPM with 4.11's doesnt mean that there is enough motor yet with a set of 3.55's.
Regarding automatics, remember a TH350 is only 73% efficient. The remainder of the 27% goes to make heat. Where a manual transmission is closer to 94% efficient. If your looking at automatics, I would go for one of the overdrive units, either a 200R4 or 700R4 and run a vacuum and brake switch to shift in and out of overdrive.
I agree though, that a 5-speed (with overdrive) is a good choice.
