Tech.....you are probably right in a fashion.....there werent enough "remaining" blocks and cranks to build 307s for all those years BUT there was tooling to build the blocks and cranks and that tooling was available and probably idle otherwise. As I remember, in the 75-78 range, there was a shortage of 350 engines in the production schedule.....you had to wait longer for a 350 ordered whereas a 307 was delivered quicker.

Both the 307 and 305 engines were offered to fil the gap between the sixes and the 350.......for economy reasons. My experience with a 305 in a pickup was that a 350 improved the gas mileage (although both were factory 4bbl).