After establishing the yoke location you can determine the driveshaft length by measuring from the centerline of the front U-joint cup to the centerline of the rear U-joint cup (dimension A).
If you don’t have a yoke, measure from the rear of the tailhousing to the centerline of the rear U-joint cup (dimension B). In this case the driveshaft shop will need to know the transmission spline diameter, number of teeth (C) and the transmission seal diameter (D). This will allow them to supply the right yoke with the new driveshaft.
In both cases you will have to supply the driveline shop with the diameter of the U-joint cup (E), the distance to the retaining clips, and whether those clips are on the inside (F) or outside (G) of the joint. You should also check both front and rear joint as they may not be the same.