Lots of companies had that rear assembly. Some stock, some aftermarket. Some worked well, others not at all.

An Aussie on Do The Ton has a frame like that, old BSA Bantam that he stuffed an XT250 engine into. Is one of the top 5 in the world in some major contest right now.

I can get you a set of dog bone risers from the indie shop with the springer. you clamp the lower bar in your stock risers, the new bar in the dog bone risers, and end up basically infinitely adjustable.

Frame stretch? Strictly to fit the tank I'd prefer to run. Use a Voodoo Vintage or Ryca rear section, set the tank how it needs to be, and fill in the gaps after excising the square frame backbone. Couple days worth of cutting and welding, week or so to fix the wiring (again). Including beer breaks...