For what its worth i found even with a 5.38 rear end and 30 in, tires shifting from 3rd to 4th with the original sm 420 required rpms high enough to scare me and end up lugging in 4th. when climbing any type of grade. I eventually switched to a 4.57 and a t5 at the same time which puts the gears close enough together to be able to shift up even climbing mild grades and keep up with highway traffic.
I can only imagine what it would have been like with a 4.10 and a sm420 but I suspect it would trade pulling power for higher speeds.
That being said my truck weighs 3 tons and has the aerodynamics of an apartment building which might effect my results.
John
