I thought Id post this recent project up. You cant find very much info on extending your own swing arms out there, heres my version. Ive taken most of this info from other writes up Ive seen so I cnat take all the credit. But Ive added a few touches that will add to its strength. Here Ive stretched mine 2 1/2" and will eventually extend the tail section the same length to save the stock shock geometry. At 2 1/2" you need a 139 tooth softail belt, at 2" you can use a 135. If I eventually rake the front 3 degrees, I will end up with a similar wheelbase of stock big twin.
DO NOT attempt this if you are not capable of welding at a professional level. This is a mod that WILL kill you if it fails. Im not going to try to teach welding in this short forum, so if your not a confident welder, stay away and hire someone to do it. With that said, lets chop this bitch!
I used a second swing arm off ebay so that I could end up with only one weld seam. These two will eventually join forces and become one.... better, stronger, longer.

You can start to see the plan taking shape. Here you can see where the cut marks show the second swing arm will be 2 1/2" longer than the first.

Tack welding a brace in place will help keep the correct geometry of the arm. A piece of all thread with nuts on both sides of the arm will also work pretty well but it can tend to bend easily. If my clutsy *** were to drop it, I would be screwed. Obviously this should be done BEFORE making cuts. I just forgot to get a picture before the cuts were made. I took MANY measurements in order to check and double check throughout the build. Warpage will be a constant issue to overcome with this type of project.

Cuts are made at a 45degree angle in order to have a longer weld seam instead of a straight cut. More importantly, less stress is placed directly on the welds with this angle.

Slugs were fabbed from rectangular tubing. This has to be fabbed because the ID of the arms tubing is an odd size. And you want it to fit snug so the arm doesnt have to flex before the slug can absorb some stress. Tubing was used instead of rectangular rod so it would be more flexible. The rigid rod would put all of the stresses and flexion at the "ends" of the slug rather than allowing the whole slug to absorb stress. I drilled holes in the arm to rosette weld into the slugs for added strength. You might notice the edges of the cuts are ground to an angle for better weld penetration.

Its all assembled here getting ready to measure a thousand times and then weld. If youll notice, I made the cut on the right side where the MOCO made thier pressed in area for sprocket clearance. Although this is a more narrow area, I was able to use a 10" slug vs about 5" if I made the cut in the wider part of the arm.

Im taking a primitive measurement here primarily to show one of the more important ones. Get this one wrong and you will be crabbing down the interstate.

Start with the rosette welds, then small seam sections, continue to check measurements as you go to minimize warpage.

Cleanup your welds, specifically the sides. Although this would weaken the welds, I plan on adding flat stock on the sides for added strength.

Here is the finished product (before adding the flat stock to the sides). I wont be adding anything on the top or bottom (of the brake side at least) in order for the rear caliper to still fit unobstructed.

Another thing to consider is that if you are planning on continuing to use your rear caliper in the stock configuration, the rear shock will contact the caliper due to its more aggressive angle. My solution is to lengthen the tail section under the seat at the shock mounting point (or at least move the mount). This will allow the top shock mount to bring the shock back to the proper angle. Lengthening the tail section will likely require seat work and repossitioning the rear fender, which I will be doing.

 

Heres the finished product, I still need to redo the seat and loose the sissy bar.

 

This thread is old but how about a follow up post? I'm curious to know how it affected handling and ride? I would think 2" probably didn't change a lot but you never know.

 

Heres the bike 95% finished. The sub frame was stretched 2 1/2" as well as the swing arm. The struts are from a doner Titan chopper, which had a great saddlebag and sissy bar set up.

The ride was much improved, felt like a bigger bike. The RK air shocks helped as well. The compressor fit under the seat thanks to the stretch.

The biggest improvement was burnouts, with the hopped up 1250 it was hard to keep the front wheel down before.

 

Looks good! The leather job is pretty cool. Post some pics when it's done