I think the auto cars got the 6.6 liter sticker and 4 speeds got TA 6.6 (400/220 hp)? I can't recall for sure anymore, and I may be mixing up years here, at the dealer the "official" way was to look at the VIN, if there was a K (I don't recall the 400 code anymore) in it, it was a 403, it not a 400. What we really did was open the hood.

The 403 was a tire smoking torque monster, pretty well out of breath by 20 MPH. The base 400 was stronger overall even if the bottom end was a bit softer. The 220 hp 400 was impressive for the time. The later turbo cars were soft on the bottom end but after they came up on boost they ran strong.

Anyway, FWIW, best I can recall 40+ years later.

edit: a memory returned, I'm only talking about 1977 and later. 1977 was the first year for 403 in a Pontiac. And Buick 350's in the big cars...