What's a 15in spindle? The pbr's should work on any spindle 93 and up with stock rotors.
The caliper brackets labeled "15" are stock - the ones labeled "16" are Sport and/or Mark VIII. I should have said '15" spindle assembly' to distinguish between those and the ones that require 16" wheels better.
To the OP - the PBR upgrade requires new brackets, but amazingly, RockAuto sells from several suppliers that supply brackets WITH the calipers (fancy that!) and also list the brackets as a separate line item.
For the early 200x GT / V6 Mustangs, that is.
They fit a standard rotor for the Thunderbird/Cougar from 93 to 97.
Caveat: Get the banjo bolt (or stick with the 2004!) since several years of the compatible calipers used a different banjo bolt thread.
As to the SPINDLES, yes, they are the same (Mark VIII and 1993-1997 TBird/Cougar). The calipers and the caliper BRACKETS change, as do the rotors, with the Mark VIII and the TBird/Cougar Sport requiring 16" wheels to clear (but being interchangeable), the TBird/Cougar non-Sport clearing 15" wheels.
The Baer 13" usually requires a 17" or 18" wheel to clear (there's some 16" that will clear.) No arguement, that's a better brake, although it also will require either redrilling the rotors to fit the Tbird/Cougar/MarkVIII pattern, or replacing the hubs with Mustang ones (the proper ones are here in posts, or can be found at TCCoA's forums, and since I'm not headed that way, I don't have the proper years memorized.) The question is, is it enough better to be worth the money and pain? Besides, to do that, you'll need a set of 93+ spindles ANYWAY - I'd grab Sport/Mark VIII spindle sets, put those on, and be saving for the Baer. Or the PBR (get new rotors when you move to PBR, so they'll fit). Or even go Mark VIII (oops! Need moar brakes!), PBR (Better, not good enough), and then Baer
Step at a time as you have money. Reselling all the stuff you DON'T use, natch!
If you're doing a Sport upgrade, and it's a U-Pull-It, grab the handicap spare out of the Mark or TBird/Cougar so you'll have a spare that'll clear for JustInCase. Or even if it's NOT a Sport set, grab a Mark VIII handicap spare so you have one, again for that Case fellow (JustIn Case, you know him?)
(I'd also have that chunk of rubber replaced- 20 year old tires ARE known to suffer from dry-rot ... )
RwP