The car is a legit J Car chassis but the price is a result of the fact that the car was one of the two J cars which was built as a can-am chassis (G7As) to little success. They were running BOSS9 engines with all the trimmings.
The can am J chassis are pretty darn cool cars and, frankly, i think it is well priced even given the fact that it wasn't originally a MKIV car. A legit MKIV should bring well over $10m in todays market, even for the "worst" race history examples I would think.
FWIW, the other Can Am spec J chassis is also alive in well in SoCal, and also bodied as a MKIV now.
Kind regards,
Vern