I worked out some numbers with the SM420 with 28" OD tires:
Speed at 1500 rpm (start of powerband)
SM420: rearend-> 4.11 3.73 3.55 3.08 2.73
1st 7.05:1 4.5 4.7 5 5.8 6.5
2nd 3.57:1 8.6 9.4 10 11.5 13
3rd 1.57:1 20 22 23 26 30
4th 1:1 31 34 36 41 46
Speed at 2500 rpm (optimum 235)
SM420: rearend-> 4.11 3.73 3.55 3.08 2.73
1st 7.05:1 7.4 8 8.4 9.6 11
2nd 3.57:1 14 16 17 19 22
3rd 1.57:1 33 36 38 43 49
4th 1:1 51 56 59 68 77
Safe Redline speed (3800 rpm)
SM420: rearend-> 4.11 3.73 3.55 3.08 2.73
1st 7.05:1 11 12 12.5 14.5 16.5
2nd 3.57:1 22 24 25 29 33
3rd 1.57:1 50 54 57 66 74
4th 1:1 77 85 90 103 116
My gut feel (with 28" tires) says that 3.55:1 is best for a 235 3 speed 1/2 ton or 216 4 speed. 3.08 is best for a healthy 235 or 261 with 4 speed. Taller tires will be better with 3.73 or 3.55. I fell like 68 is a good cruising speed for me, and I think 2.73 would be way too steep for hauling a load or pulling a hill but you can downshift to third below 65 if you have to. I don't think I need to go 74 in third gear. 235's can rev to 3800 regularly without damage but their life is short if run that way for sustained high speed.
