Found nothing on a search....

Got the 454/th400 on the frame yesterday. the body is up on blocks at the moment. Is it imparative that i have the whole weight of the vehicle on the suspention before i measure and order a driveshaft? how much of a differnce does it make? Just wondering if anyone has experience with this or if i have to wait till everything put back together for this. I was hoping i could put in the D/S and exhaust without the body on, make it a little easier on me. what yall think?