I could be wrong, but my recollection for the axle width of a 71-73 Mustang is 58", not 61. I woould recommend verifying any information found online. Contrary to popular belief, just because it's on the internet, it doesn't always mean it's true. LOL
All that said, that Boss is worth more complete than the sum of it's parts. It's a shame that one is being parted.