AT 545 and 321 rear gears

Allison man; "I have another project" I currently have a argosy motorhome with a 454 and th400 and 4:10 diff. I have a AT545 with a chevy adaptor. When we looked at the gear ratio of the first gear in the allison it is so low that if I installed 321 gears instead of the 410 I have now I would still have a lower first gear than I have now with the th400. My thought was to end up with an overdrive out of the deal. Let me know what you think.
AT 545 and 321 rear gears

Hi William,
Overdrive is not possible with that trans. Please do not add on any overdrive kit. This will damage if not break off the rear housing. An AT545 has a high range ratio of 1:1. The new electronic LCT1000,2000,24000 does have an overdrive in it. It replaces the AT. You can drive one if you want, just go to a local chev or gmc dealer and test drive one of their new pickups. The problem here is it is hard to find a surplus LCT and you need a computer and wiring`. Good luck and have a great day! :)
AT 545 and 321 rear gears

Hi Allison man; I was probably not very clear on what I wanted to do. I currently have a th400 behind a 454 chevy in a 28ft. motorhome with 4:10 gears in the differential. What I want to do is put in 3:21 gears in the differential and install the AT545 I currently already have. When we compared the gear ratios between the first three gears in both transmissions the allison will still be lower in the first three gears with the 3:21s than the th400 is with the 4:10s. Thus giving me a 20% overdrive when I shift the allison in to 4th gear. See if this makes sence.