1st gear in a TH400 is 2.48 and with 3.08 is 7.63 overall ratio. That would be like starting in 3rd gear all the time with what you have now, if you have 4.56 or in 4th if you have 5.13 rear gears. But you have a sm420 with a 7.02 1st and a 3.57 2nd gear. 1st gear with a 3.08 would be 21.62 and second gear would be 10.99. With the 3.42 gears you get 24.00 in 1st and 12.20 in 2nd.
Your sm420 has 7.02, 3.57, 1.70, and 1:1 in 4th. Multiply each of those gear ratios by the rear gears that you're considering and compare it to what you have now and how you use those gears.
