I have a few pics but I'm afraid I don't have the patience or foresight to document my life. I am a bit of a mad scientist when it comes to these types of things.
Here's what I did, I hope it is understandable.
I mounted the bike to a 2x12 and secured it.
I then cut off the rear half of the frame behind the OE down tubes, leaving the tubes, footpeg mounts, brake pedal pivot and swingarm pivot in tact, like 77hardtail did.
I then built a jig out of the old swingarm uisng 1" square tubing, extended 5". I made sure it was nice and square and used a piece of heavy threaded rod and 4 nuts to hold the OE axle plates in place. The jig pivoted on the original swingarm pivot, which allowed me to adjust the hight of the alxe plates, plus it was a solid, square spot on the OE fromat to work off of. I would not use the OE axle plates if I did this again. A lot of extra work and they are not right for the job.
I measured everything again to make sure it was as square as I could get it.
I'd like to say I had a blueprint and exact measurements to give you for the hardtail tubes, but the truth of the matter it was done by eye. Single bends in the tubes are not hard to fit though. A double bent tube would ahve to be more accurate. With the single bends you can twist the tubes a bit to change the angle. I just made sure I had a piece of DOM long enough for the job and estimated the bend angle to start. I did one side first, making small bends and placing the pieces as close to where they were going to be as I could get them until they were proper. When I was satisfied, I laid the pieces on a scrap piece of plywood and traced them and built the other side to match. I happen to have access to an exhaust bender, which is not ideal but it worked fine for the slight bends I needed.
I fit the bottom tubes first, and notched them so that the OE frame cradle tube sat inside of the notched lower hardtail tubes. For these notches I basically cut the tube in 1/2 for about 5". With 1-1/8" DOM the cradle tubes seem to fit snugly, but will need to be heated and adjusted for a snug fit as you weld. This method allows you to do a lap weld instead of a butt weld, and you don't have to modify the OE frame. You will probably have to cut the kickstand off and move it forward also. I also had to make sure the new sections fit inside the slots where the OE swingarm tubes fit the axle plates. Again, DON'T use the OE axle plates. It is a PITA.
I then found some round shaft that fit snugly inside of the smaller backbone tubes to use as slugs. I bent them to the proper angle and found a small piece of tubing that fit snugly inside of the new hardtail tubes and that the plug would fit inside of. I welded these pieces together and drilled holes in the OE frame and the new upper hardtail sections for plug welds. I cut and notched these sections so they fit the axle plates and the slugs, and to fit cleanly with the OE frame. I built triange gussets to fit at where lower hardtail and frame meet.
After another million measurements to make sure everything was square, I tacked everything in place with a little 120V wire feed welder. I installed the wheels, etc to make sure everything fit I then took the frame to a friends and welded it up with his 220v MIG. I also welded in a cross tube ahead of the rear wheel top and bottom
With this method most of the front half of the frame left intact. Only the 2 smaller backbone tubes are cut, and of course the rear seat and frame section. But none of these cuts will affect the strength of the OE frame.
For the footpegs, I used the lower footpeg bolt as the upper and built a new lower footpeg mount. This allowed the peg to move down and forward a bit.
I cut off the upper peg mount and plugged and welded the swingarm pivot holes. On the left side I welded a pivot for the foot clutch. On the right side I wanted to use the OE brake lever and assembly so I built an extension that relocated it lower and forward a bit.