305-4 was the Trans Am series engine size while the production T/A and AARs had 340-6.
NASCAR was trying to get rid of all of the "specialty" cars. If it won, it probably would have gotten saddled with more restrictions or outright banned.
Interesting article that mentions Petty looked at it but figured the
small block would't be competitive anyway. But obviously the team, along with Keith Black, tweaked the combo enough to surprise everyone. Ironically, NASCAR eventually reduced the size to 358 and Mopar ran their 355 small block. (I think Mopar ran a 426 wedge for a short time after the HEMI, but can't find info at the moment.)
Aero Warriors - The Little Engine That Almost Did