It's a good question, MF. The only rationale I've ever heard was they ran out of 47s. In the "pecking order", the Firebird "400" was THE performance model made in high volume. The 350 cars have always been considered "also rans" or "poor man's 400". Not literally, of course, but the general attitude among Pontiac peiople. The 350 has gained popularity in recent years due to the perceived rarity of 400s (not rare at all!).