Can't say 100% sure, but what I recall, the 403 was already approved by CARB (California Air resource Board) and the Pontiac was not. It was less expensive for GM to use the line data in other assembly plants, than to recertify the Pontiac for California. At the time, they weren't too concerned with selling actual performance cars as they were selling the "image" of performance. That's was the great era of "sheeps in wolf skin". You remember. All the "scoops" weere closed up, cataytic converters were choking things, vacuum lines everywhere... Hey, Chevy even got into the act with a "NACA" or "Douglas" scoop that was nothing more than a sticker on the hood of the revived (due to T/A sales) Z/28.
In '79. my brother and I went to various dealerships and test-drove all the "performance" offerings. The ONLY car we found rfom GM that would truly turn the tires over from a "standing start" were the T/As. But we had to go to Arizona to find them! A stock 403 car came with TH350, no other option. It wouldn't turn them. A '79 Corvette in Cal, also had TH350 as the only trans. It too wouldn't turn the tires over. -We DID find a 400/4-speed T/A at Mecham Pontiac (yes, the infamous governor, actually pretty nice guy and honorable business man) in Glendale, AZ. IT would rip the tires! Nobody had a '79 Corvette with a manual trans available for a test drive.