Setup is a 70 olds 442 with 455 with m20 manual trans, rearend housing is 70 gto.

the drive shaft that i have measures 55in. center ujoint to center ujoint

now from what ive heard is that manual trans had 56in or so propershafts.
now if that is true could i get away with using the smaller one rather than buying/ having one made?

 

Measure tip of Tran seal to center of u joint on pinon yoke with car level at rest.then check driveshaft ,1 inch of free play on trans yoke from all the way into trans pull back one inch mark with sharpy this is where the seal will ride .,to center of back ujoint.if you need that one inch free play too little space driveshaft jams into trans and bends the shaft.too much it could fall out or wobble. Btw stock driveshafts are . .065 tube thickness you can have one built out of .095 much stronger or go up in size diameter.

 

unable to measure in the form that you described as it is totally disassembled, it would require a pinion yoke cause the one that was on it like the shaft it self was hooked to a auto. trans.