i don't know how long it needs to be, the one i have now i made from my old driveshaft because the slip part was all wollard out, i just cut the ends off and welded square tube to both ends that fit inside each other. well now the cv is all jacked up.
the truck has a 700r4 in it it also has a 6" lift i guess i can measure the old one, but i'm pretty sure it's just stock length