When I did that swap I actually found the correct one in the junkyard, but that was 30 years ago... they are like hen's teeth because very few cars had an 8-3/4 behind a 904, and that was only from 67 to 69.
A few months back I bought a slant six, 904 car that had an 8 3/4 in it. Yes very strange. Either a special order or they ran out of 7.25's that day. Never found a complete build sheet to confirm but it appeared to have never been apart....same with the rest of the car. I found two build sheets and BOTH were missing the part with the axle code.