I have never welded the rear differential axle tubes myself. But I have done a lot of welding. I would use a long section of angle. This will act as a V-block. The most important thing is to have your measurements correct. You just can't whack it off with a hack saw. Your cuts need to be in the correct spot. It would be best if you could cut them in a V shape. Having your cuts, and weld, in a direct line to the application of torque will give you problems.
How I would do this is mark a spot four inches to one side of the length you want. So you have two lines going around the tubes four inches apart. Then mark the top and bottom of the tube at one line around the tube. This will be where the pointed end of the cut will be. Then mark the front and side of the tube at the other line around the tube. These marks should be 90 degrees apart from each other. Then use a bit of string to make your line around the tube from point to point/ line to line. Put one part of the string at the top mark. Put the other end of the string at the middle mark ( front or back doesn't matter) Then you an use a spray can of paint to show where the string is. Don't go crazy just get paint on the area where the string is. You might even mask off the other areas. Then use the same process to complete the marking of the tube. Do the same for the other tube. I assume you are shortening the tubs so mark the line you want to weld to and then mark four inches in from there for the the other ends of the points. Then replicate the points of the tube where you want to weld. Keeping the dimensions correct is critical. One of the ways to do this is use a length of bar stock, say half inch thick. Clamp the bar stock on the tubes so they are across the cut out area. Drill through the bar stock and the tube with a bit that is the correct size to tap for a 3/8 bolt. Tap the holes in the tubes and drill out the bar stock just enough to get a 3/8 bolt through. The bar stock will be your gauge. Now move one of the holes, means drill another hole, towards the other the amount you are going to shorten the tubes. After the cuts you can rebolt these sets of bar stock onto the cut off tubes after cutting, using the closest set of hole, and the tubes will be the correct distance apart. It's best to make two gauges for each tube. Make sure you mark them so they can go back on the correct set of holes. When you make them place them so they will not interfere with the angle that will be used as a V block for welding.
Now you are ready to cut the tubes. After you have both tubes cut and shortened place a length of angle against the tube. This is now a V block the aligns the tubes. Bolt your gauges in place and clamp the crap out of it.
When you weld you need to start with a good clean surface, no paint and no oil or grease, and the edges should be chamfered. Also you need a gap that is about the same distance as the thickness of the tube. I'd guess about an eighth of an inch would be good enough. AT this point don't just start welding it together. Heat makes steel warp. So you tack a one spot. Move to the other side of the tube and tack weld again. Just about a half inch to an inch. DO this at four places around the tube ( the points are good spots). Now you should have four inches of weld holding the tubes together. MAKE SURE THE DIFFERENTIAL DOESN'T GET DROPPED OR BENT IN ANY WAY. Those four one inch weld are now the only thing that is holding the tubes in proper alignment. Don't support the differential by having a block at one end of the tube and another near the gears. This will warp the tube. It would be best if the tube is pointing up in the air so nothing is putting stress on the tack welds.
Again clean everything. Pain, oil, grease anything other then the parent metal and the rod in the weld will weaken the weld. Now you weld. But remember heat causes steel to warp. So weld on section then move to the other side and weld that section. There are four sections so weld them up and you are done. Myself I would do this one side at a time.
By making the cuts triangular in nature the torque that is transmitted in the tube is never in line with the weld. And because it is triangular the amount of weld is almost twice as much as if you just welded it straight around.
Here is a welders trick. Preheat the parent metal, in this case the tubes, before you weld. A good gas torch with a rose bud tip does just fine. Get it evenly hot all the way around. And I mean HOT. It doesn't have to glow but it should be close. This will make your weld much stronger because the difference in heat from the weld area to the none weld area is not so great. And with the parent metal preheated the bead is very nice when you weld. Make sure you use new rod. Rod that has been sitting around sucks water out of the air is makes for crappy welds. In production shops or big job sites they have an oven to heat the rod and drive out the water. But for an individual just buy new rod.
I know this sounds like a lot of work but it's really not. An it it keeps you from having the axle tube break while you are going very fast it might just save your life. You do know that things like this only break when you are stressing them with speed. So good luck.
Do it like I pointed out and it will last forever. Cut it off straight and weld and it will probably break after hitting a few good bumps in the road.
anybody in Tampa have a jig to weld 8.8 axle tubes
