Thom, I also have a 6 cyl with 3 on the tree in my wifes C10. I'm pretty sure the gearing in the rear end are installed for max torque for those tree stumps that needed moved (my truck was a real farm tool). I really want to leave the 6 in and change either the tanny or rear end gears.
A good 6 can get you the speed you want.
See if this gives you any help:
I haven't dealt with this company, just found him on the net.