Let's discuss dual stage injectors separately, I have them installed in my SD.
Language is German.

The reason why I don't like your plan of new thread on 617 injector body is that you can not use the heat shield plates any more. Means these you also need to do custom.
Can't you just cut off the upper part of old angled 60x PCs and weld them to the rest of the cut 617 PC?
I made a thread ring with the 60x thread, machined off the thread of the 617 body as well as a step and pushed the ring on, fixed it by welding.

Tom
(this was how I put dual stage injectors to a angled PC).