I'm curious what a Boss 429 NASCAR Mustang is?  The Mustang never raced in NASCAR because it wasn't an intermediate body car.  Ford ran the Torino in NASCAR.  The idea of the BOSS 429 was that Ford wanted to homologate the BOSS 429 engine for NASCAR use, but knew they'd never sell 500 examples of a BOSS 429 Torino.  Exploiting a loophole in the rules that did not specify that the 500 examples of the engine had to be installed in the same chassis Ford decided it was easier to sell 500 BOSS 429 Mustangs a year than 500 Boss 429 Torinos.  This allowed Ford to race the BOSS 429 in the Torinos, because they had sold 500 examples of the engine, and 500 examples of the chassis they wanted to use.  The BOSS 429 was an option on the Torino, so all the specific installation parts had factory part numbers and were legal, but my understanding is that only a handful were actually built. 

I used to think the BOSS 429 was an amazing car capable of amazing feats of speed until I talked to a guy who actually had one.  He bought it for next to nothing during one of the gas crunches.  He wanted to race it, but couldn't believe that in stock form it was turning mid 14 second ETs.  See, the cars were built to make the engine legal for racing, but it required shifting the engine forward a good bit to fit the Mustang engine bay (even with all the shock tower mods).  This made the car dangerous from a handling standpoint, so Ford severely detuned the engine in an attempt to make it safer.  In stock form the car was a bit of a dog, especially for what people thought it would be.  They took quite a bit of work to really make fast, and that required knowing the right people because all the speed secrets were pretty closely guarded, at least back then.  It was an interesting tale, and the car had long since been retired from racing as the guy's current race car was both not worth anything and much, much faster ('81 Mustang with a 351C that ran in the 8s). 

I still can't fathom the prices these cars, and their parts bring.  I'd much rather have a car that I am not afraid to drive, handles better, and is faster by using modern parts. 

Nascar Boss 429 was designated as such for being the first ones produced.  I don't remember all the details, but there were subtle engine differences.  Certain serial numbers.  Of course none ever saw a NASCAR track, but that is how they are denoted.
And to top it off, that export brace won't fit anything but a BOSS 429 with the modified shock towers.  Now, if you are converting your old T code with BOSS9 type shock towers, that is the part you need!

I knew better than to say anything...

I got $20 that says he won't.