Tony,
No I did not use the bumpers mentioned by the OP. I was lucky enough to find a set of original EURO bumpers and hinges.
As to the mounting holes on the hinges, as I remember, the holes were configured a bit differently but at least two did line up with the frame mounting holes so I called that good. My car is a Series 2 (1976) so might be different than yours. In your case, if the holes don’t line up, I would think it would be much easier to drill the hinge box rather than the frame. The metal is not as thick and you could use your original hinge as a pattern.
Sorry I was not much help. Hope you get it to work.
PS, If you need to score the euro style front turn signals,there is a guy on this site that has a pair listed. Check the classifieds section under, XJ series one, two and three parts for sale .XJ Series I, II III parts
Good luck,
Phillip