driveshaft length rear diff swap

Since you're a welder... If you know how to properly seperate/grind the end-piece/yoke from a driveshaft and be able to shorten the remaining shaft nicely with say a pipecutter, you could do all the work yourself. Save for balancing usually.
Make sure to keep the yokes on both ends of the tubing aligned, as they are now.

If you weld cleanly you probably could drive the car as-is without balancing, but having it balanced is advisable.

I've shortened the same driveshaft in my '73 Dart twice myself and still can get the car upto 80mph without serious vibrations.
I first went from a 7-1/4" axle to a 8-3/4" and later on from a 904-transmission to a A518.