Still pretty big difference in the lowers to uppers. If anything you want your uppers longer than the lowers so your pinion will rotate up during droop. Longer lowers will rotate your axle down during droop,and most definitely cause drive line issues.
I also clear 39" tires at my height at full stuff full lock with minor fender trim at the cab. I suggest you move the axle forward at least 6" from stock if you want to clear 42's eventually. Mine is about 3" forward, and i can't clear my 40's at full lock full stuff, there's no more room to clearance into the cab. Also, my frame is 19" from the ground at the trans c member.