There are many posts on driveshaft lengths and the end result seems to always be - measure once everything is assembled to be 100% sure.
Better to wait than to waste money.
THIS---------- With it all together, sitting on the ground. take measurements to driveshaft builder w/ info on how u measured it. do it once-do it right. If in doubt on how to measure it, ask the guy that will be building it, how to measure it.