G'day Nic
Yes those are for the pillion foot pegs.
I've done some searching but I can't find that dimension. Suffice to say they should be equidistant to the swing arm. I just measured mine to be 8mm gap from the s/a to tube going past.
My 57 frame was also butchered coming from South America with sidecar lugs welded on the wrong side for us. I just tweaked them to what looked right.
Now get over to "Introductions" and give us your story.
Cheers