This is all great info but to go through all of this to change from a 4:57 to a 4:10 seems like a lot of effort for little gain. Presuming you are not referring to 60's trucks with coil springs. How about changing the entire rear end housing. It seems to me that there would be a number of later model trucks with better gears in wrecking yards all over. Of course you would have to get one with the correct width and reposition the spring pads but you would be getting a more modern set up.
Fred
