Hi Tim,
I have checked around and there are very few shops that want to mess with the C5/6 vette driveshafts. It's made of an Aluminum Metal Matrix Composite material. That requires a special welding process to fuse it with the proper strength. The shops that have said they could shorten them - either weren't aware it was AMMC material, or they didn't have the balancing setup required to balance the deal properly.
You'll run into the same deal above - plus you're up against lengthening it which could get you into a harmonic failure mode. So I would recommend against attempting to lengthen a stock unit unless you can find a shop that really knows what they are dealing with.
I'd replace it with the correct length with a new unit. I know of 2 manufactures. 1st is
www.driveshaftshop.com They custom make the ends and tube outta 6061-T6 aluminum (not AMMC). The 2nd is outta composite material from LG Motorsports.
www.lgmotorsports.com A couple other builders here have used this product to extend the driveshaft so they might jump in and tell you about there experience.
Good luck !! Jim