My '51 chassis is sitting in the yard. I could measure them for you. Mine are cracked as well, but the bolt holes are still there.
But the FAM has dimensions of the holes. 8" between holes, which are 7/16" x 11/16" slots (long dimension front to back). From your pic, it looks like the front of your slot is still there, so you could go by that for the front-to-back location. Section 2, Sheet 2.00 if you want to check yourself.
