I don’t know t this has been a long thread blame me and Taylor, he started another post on the ringer lolHello All, please correct me if Iḿ wrong; I have a convertable 66 4spd sitting on the back burner for about 10 years now. Had a machine shop redo the engine, and they informed me the car has a 421 I asked the mechanic who was doing the rebuild if the engine ever looked like it had been changed, and he told me not that he could tell. (that was about 15 years ago) I later replaced the clutch and had problems with Summit Racing (I think) because they had no listing for a gto with 421. I then started to do research, and found some literature about Pontiac running NASCAR Catalinas with 421s (I believe there is one or two valuable aluminum body models floating around somewhere). Anyway what I further discovered is that the 421 and 389 blocks are the same in that, intake, exhaust, etc can be interchanged. My car (here in Hawaii) was ordered from the dealer (i have the build sheet) with factory air. Now why would you need factory air in a convertable? Because at the same time the car was ordered, a 421 shortblock was ordered. GM would not allow an A body car to ship from the factory with a engine larger than 389 in 1966. (which may explain why they relented a litlle in 1967 due to competion with other car models, and shipped GTOs from the factory with a 400) Anyway, from what I have gathered, you walked into the Pontiac dealer, ordered your car with AC because an AC car has an oversized radiator, and at the same time ordered the 421 shortblock and paid the dealer for the swith out, so when the car arrived, the engine was yanked, exhaust intake accessories etc were switched to the 421 block and reinstalled in the car. When your car was delivered to you, you drove off the lot with a "dealer option" GTO with a 421, but the emblem badges were not changed, fakeing out any competitor in a stop light off the line challange. (unfortunatly the previous owner did not order power brakes which makes stopping interesting). Please update me if I have any of this wrong because I did spend hours of research trying to get to the bottom of why my car has a 421; btw the casting numbers on the block, exhaust & intake of of the same year series making unlikely the idea that someone (my car was a one owner) could go to the junkyard in later years to buy a 421 & find a matching casting series.