The SM420 is kind of unique among four speeds in that it has some really wide spacing between gear ratios.
There is a fairly big leap between 3rd and 4th, for instance. If you set it up so that you can actually use first gear on the street and make a quick jump to second it won't be so bad, I suspect.
I made a excel chart of engine RPM versus speed that I can email you if you are interested. I don't have a web host set up so I can't post it, but maybe someone else can.
I have decided that I want 3.08 gears with 28" tires with my 261, giving me a nice cruising rpm of 1980 RPM at 55 mph, 70 mph at 2500 rpm, and the ability to use 3rd up to 60 mph.
A 3.55:1 will give 2900 rpm at 70 mph. 3.40 wouldn't be too bad, the differance might be about 250 rpm at 70 mph between the 3.08 and 3.40, which isn't going to break the engine.
The gear ratios of the SM420 are:
1st- 7.05:1
2nd- 3.55:1
3rd- 1.55:1
4th- 1:1
