Spencer,
I did an overdrive transmission swap in my 59. My rear axle was leaking so after the transmission I had the axle overhauled, and the gears changed from 3.90s to 3.38s. I should have kept the 3.90s as my truck has a flat spot around 2000 rpm. In short The 3.38 gears are almost to tall to use with the overdrive.
My truck before would do 65 mph at about 3150 RPM
with the overdrive it would do 65 at 2300 RPM
With overdrive and gear change I can do 65 at 2000 RPM, but it has to be pretty flat.
I have the stock V8 in my truck, and not the 6 banger. If all you are doing is swapping the gears, I think you will be pretty happy. Changing gears from 4.11 to 3.55 would effectively give you an OD ratio of .86. Or in other words your RPM will drop 14% in your top gear.
John
