When I get back to it, I will check the back-rod. I do have have the timing-marks at the "crank-end" of the machine ( a good question though ! ), and I found the dimples in the rod from the grub screws, and tried to line them up with the holes before installing / tightening the grub-screws.
That's not to say there might not be a slight "smile" in the back-rod.
I'll have to see if I have some really good, thick plate-glass that I can use as a surface-plate, and give the back-rod a roll and see if it's "lumpy".
Regarding the orientation of the half-nut / spring to the the feedscrew: I assume the threads on the feed-screw are "pitched", and that the long-edges of the spring should be perpendicular to the feed=screw ?
I guess the "Test of tests" will be to have this beast pull its way through cylinders while "dragging" a 14" witches-hat.