I admire the solution.
However, is the "old" OEM fuel pump really "old, fragile and expensive?" I'll give you the expensive, nothing with the Star or Bosch name is cheap. But, with reasonable care, I think these pumps can last about 30-40 years. That's nothing to complain about. It garners complaints, when its end of life appears when in your hands…
And, if you just get an old fuel pump from a 124, then do all this work on it to make it all work, what exactly, is the point? Sounds like you are adding a lot of custom stuff, and who says the 124 pump is any good if its on a junkyard car? You might (not will, but might) just be creating a lot of work, with no net gain.
I like the idea of brazing a fitting on the end of the fuel pipe. Is it possible to find a proper compression fitting, and avoid the brazing? Just a thought…