Shortening a drive shaft

when building a car from mixed pieces or after doing and engine-trans swap you usually need a new drive shaft. drive line vendors have info posted on how to measure the length you need. usually install the trans yoke all the way in then pull it out 1/2 inch then measure between U joints. For the The next step I get out the dial calipers , measure the U joints required then head to the parts pile or the the local wrecking yard and start measuring,  There are conversion U joints that have opposite sides  different sizes,  I check out the rack and also don't forget to check out light and medium trucks. I have found 1/2 of a 2 piece truck drive shaft that fit one of my project. If you don't find one to fit then look for a longer one with minimun balance weights. If the factory got it close enough with out weights you will have better luck getting it balanced. I lay the drive shaft on the work table then align a piece of angle iron next to it and scribe a line all the way along the shaft and get good lines on the end yokes. I figure out how much needs to be removed then chuck it into the lathe and cut the rear axle end part way thru on the factory weld. , Next measure  and make a scribe cut at the proper place. Next I polish the area next to the cuts so I will have a good surface to weld, I like my home made steady rest, wood V block,  an old leather belt and grease. I make the finish cuts going slow, Next I align the scribe marks, tap the shaft together with a plastic mallet then make tack welds. check, then finish weld. I also like a drive shaft safety hoop, just in case. I have never had a drive shaft break, even after a hard wheelie bent the rear wishbones.