IF, the place you got it from won't get you the right one (which they should!) I would just take the measurements from center of U-joint to center of U-joint, and take the whole set up (U-Joints, Yolk and shaft) to a local drive line shop and have them shorten it.

I had a project that I swapped a TH350 for a TH400 and had to have a whole new shaft made.. was a little pricey, and had to have them shroten it slightly after it was made... but it worked out ok...

But I'd get after the company that made the shaft.

Brett - 67 Drop Top 455!