Determining what to take out: slide the yoke end of the drive shaft into the tail of the trans. Then pull it back out about 3/4 of an inch (that's what she said). Now measure from the center of the eye (the part of the joint that gets pressed in) on the u joint to the center of where the eye of the rear joint would be when bolted to the rear end. you now have your new drive shaft length. Now subtract that from the length of your current drive shaft. Thats how much to cut out.
Ok here comes the fun stuff! You need to scribe a straight line down the drive shaft. This line will allow you to keep your yokes in exact factory phase when you go to weld it back together. If you get the yokes out of phase you will end with vibration. Get out a metal scribe and a long straight edge. Put the straight edge down the length of the driveshaft centering it off the same u joint centers you used to measure the desired length earlier. Now choose where to make your first cut. This is up to you. Perhaps you want to retain a factory weight or get rid of a dent. It's up to you. Remember to take your time and get a straight cut. This helps a lot when welding it back together. We used a gravity style band saw
Which made for a very precise cut. We made our first cut about ten inches from the yoke. We needed to remove 25 1/2 inches from the shaft. Here we are measuring from the first cut to the Blade. Now would be a good time to double check your measurements.
Next we beveled our edges to give the weld a little v shape to lay in to. This helps with penetration ( that's what she said).
Now we take both pieces of the shaft and clamp it between two small pieces of u channel. Make sure you line up your scribed line and of course strip the area where you are going to weld.
If you look closely you can see our scribed center line.
Now place a few tack welds around it. Then finish up with a solid bead. Make sure you ground your welder to te drive shaft and not a u joint bearing. You can ruin the bearings. Putting in new bearings you say? Well then ground wherever you want.
Now grind down your welds. Don't go nuts. Just get it pretty smooth and be done with it.
Next check the fit in the car. This is a 34 ford so we had a pretty short length. 33 1/4 inches to be exact.
She fit like a glove! Now we will take it out and put in new u joints and have it balanced. Then we'll paint it. It's up to you whether you have it balanced or not. As long as you kept your yokes in phase you should be fine. I suppose it depends on the project and your pocket book. Balancing usually costs around $150. Give it a shot first and if you don't have vibration at highway speeds you did well. Good luck and be sure to wear safety equipment when working with power tools. Work smarter not harder.