the other alternative is the Chrysler 9.25" diff . It was used in "B" Bodies starting in 1973 , and was last used -- iirc -- in 1980 , 360-powered "R" Bodies ; they're still used in Dodge trucks ; parts are plentiful .
IF you find a "B" Body in the salvage yard , and it's a '73 & up , and it's got a 400 HP or 440 , it'll def have the nine-and-one-quarter diff in it . It'll also most-likely have 2.93 - 3.21 gearing , but may also have 3.55 ( less-likely in non-HP applications ) .
While the 9" is undeniably a stout piece , they're also uber expensive to build . There were different spline-counts , with the HP ( e.g. , axle package / towing package ) axles being the most sought after ( Mustang / Cougar with 3.90 - 4.30 gears ; Rancheros with 3.50 gearing ) , which -- iirc -- is the 31 spline deal .
Off topic ( slightly ) : back in the early-mid 90's I had a 1970 Ranchero GT with a Cleveland 2 Bbl / FMX combo , and a 9" with 3.50 / Locker .
The driver's side axle shaft was bent (!!) when I got it ( the car was perfect otherwise ) as the prior owner slid and smacked a curb ( he was older and not flogging the thing ).
I searched high & low for a 31 spline replacement , to no avail .
Finally , by the grace of God , I found a '72 Ranchero in the 'yard with the same setup
( 3.50 / Locker ) . If I had to buy a new / replacement one from an outside source like Currie , I'd have been screwed .
Nevertheless ...
the 9.25" Chrysler rear is another alternative for you .
Good luck with your venture .
