Speaking of the Monster Miata, here's another view of an authentic Monster with the Ford diff added.
If you are buying a Monster, having this part is the sure way to get both durability and a reasonable RPM. The original Miata or RX-7 gears are in the 4:1 range, and that kind of gearing with the Ford T-5 (whether it's 3.27 or 2.95 first) is silly.
The diff is available in 7.5 or 8.8" size, aluminum or iron casing, and with or without a limited slip. The variations occur depending on donor car: SVT Mustang Cobra (IRS), Lincoln Mark-VIII, Cougar, or Thunderbird.
It's also available in aluminum 3.783:1 8.8" form brand new from the Ford racing parts catalog.