I've shortened driveshafts before. I just did one for my '52 3600. I converted the front driveshaft to open driveline for my heavy duty 3 speed.
Photo I shortened a front shaft from a 1 1/2 ton.
Do not cut the shaft and try to weld it back together. Carefully remove the yoke, cut the shaft down, preferrably with a pipe cutter, insert yoke and measure from the other end all around the yoke to get it straight. Tack one spot, move 180 degrees, measure again, tack, etc. My shaft is only about 30" long. I haven't run it yet, but I don't anticipate any problems.
Don't take this the wrong way, but if you don't consider yourself at minimum a "good" welder, don't try it. If you don't have a 220 volt welder, don't try it. I wouldn't screw around with a longer drive shaft either, best to let a professional do it.
