I JUST got done moving the spring perches on my Dana 70hd that came from a 96 3500HD Chevy. The spring perch center distance was 42.5 inches on my 1950 1.5 ton, and the spring perch center distance on the donor Dana 70HD was 40 inches. Using Grigg's advice, I just took a grinder to the spring perch weld beads and worked them down good, then commenced to chiseling them the rest of the way off. Just make sure you don't grind down into the axle housing much.
It took a while with a chisel and hammer to get them knocked off, but after that, it was just a matter of grinding the weld beads the rest of the way off the axle and perches, and getting the pinion pointed at the correct angle prior to welding them back on. There's an article in the tech tips section
http://www.stovebolt.com/techtips/driveline_setup.html for setting up driveshaft angles, so it's relatively straight forward.
