No Mustangs, that I'm aware of, were ever entered into any NASCAR race, ever, during the 60's/70's. They were built to homologate the ENGINE for NASCAR racing in Torino's/Marauders, the mid sized cars used in NASCAR races of that time period.
I doubt that the seller is trying to say that this is a NASCAR "RACE" car. He is probably referring to the engine designation, early Boss 429's had internal differences, (S code) vs later engines with smog and different cams (T and A code). However, being a 70 model, I doubt that it has the early S code engine. Seller may not know this or perhaps the engine has been changed/modified, or he's making it sound better in order to sell it. Kar Kraft modified/built every Boss 429 for Ford.
You'll need to do more research in order to determine the actual engine code. You need to be aware that this car is far from a "stock" Boss 429. It's value isn't anywhere near a stock, correct Boss 429 Mustang and I doubt the engine code is going to have any effect on the value/performance of this particular car since it has been highly modified.
The Marti report verifies that the car is indeed a genuine Boss 429 Mustang. This car has been highly modified, it's far from stock, and that's the reason for it's value, most are well above 150k...