rearend pinion sadle to seal on trans is 57 3/4 inches. what size driveline not counting slipyoke length.

Try this for the answer (http://www.dennysdriveshaft.com/html/how_to_measure.html)

I just had one shortened last week for a friend's '69 Firebird 400/350 to 455/400 swap. We used my old '71 C/10 driveshaft, since it had the 400 yoke on it, and it was larger diameter, and had it cut down 3/4" shorter on the u-joint centers than the old shaft, since that's the difference in case length between the two trannies. Worked fine.