[Motorhead6]

If you are running 3000 rpm, the 3.73 will drop it to 2722 rpm. 3.55 will drop it to 2591, a better road gear but still turning about 3300 at 70 mph. 3.00 will turn 2788 at 70 mph. This option would require a rear end swap to a later model 8" as 3.30 is the highest ratio you are likely to find in the early rear end. Maybe the OD transmission is the way to go if you are planning road trips.

Math is simple on this. Divide the new gear ratio by the current one and multiply the current rpm times the dividend. 3.73/4.11=.9075 .9075 X 3000=2722 rpm at 55 mph using your numbers.

70 mph divided by 55 mph gives the multiplier for increase in rpm as speed increases. In this case, 3000rpm X the factor of 1.2727=3818 rpm at 70 mph with the 4.11 gear.

Try it for yourself and verify my numbers; we tend to fat finger numbers as we get older.

As to the question of the '56 and earlier rear ends with A/T, the ratio was 3.30 at least in '55 and '56 with the Y block except in '56 with the 312 engine there was a 3.22 (close) ratio but these are rare now a days. To the question of direct fit, yes but there were 2 spline choices coarse and fine (don't remember the count). If changing the differential, make sure you get the axles too if the splines are different.