I agree but I emailed a person at one of driveshaft vendors to ask about possibly getting a driveshaft that will work for my current Dana 35 but still be usable when I upgrade to a Dana 44.
This was what they said:
If you order the shaft with the X spline and XC slip yoke there will be enough compression and extension on the shaft to accommodate minor changes in running length. I’d measure with the dana 35 and then subtract an extra half inch from the drive shaft length you calculate you’ll need with that dana 35 axle. The Dana 44 is about one inch longer than the 35 which means the drive shaft would technically get 1 inch shorter. By subtracting a half inch from the dana 35 measurement you’d end up with a shaft that is a half inch off now and a half inch off later. In either scenario a half inch is negligible and not enough to affect the operation of the drive shaft.
So I'm thinking I need to get the SYE installed and then get an exact measurement.
I'm sure I outed the vendor with that