1973 duster wheel bolt pattern

So we are installing the 8.75 rearend we got out of a '71 340 4 spd duster and putting it in our 73 duster big block project. My baffling moment is this. The 71 rearend is a 4"sbp, no suprise. I had assumed the 7.25 out of the slant six 73 duster would be a big bolt pattern. Out of curiosity, I measured the bolt patterns from both rearends and they were both sbp. They were both 4" bolt pattern. Am I wrong in thinking the 73 was a big bolt pattern, or did they just use up what they had on hand that year before going to the big bolt pattern on cars built that year? The fronts are 4 1/2" big bolt disc. What the heck is going on? I guess someone could have trashed the old 7.25 and swapped in an older 7.25, though with as beaten as this car was when we got it I don't think anyone would have fixed it if it failed. They would have left it for dead. Any thoughts?