351 4bl VS 351-CJ. Which one is it? Like Area 51, this discussion has had much air time over the years.
The difference....very little.
It was all a name game with some creative terminology by Ford and the other manufacturers. The engines did receive engineering upgrades to make them compliant with each current model year's emissions requirements. The '72 Q engine content never changed in '72, which is also when the engine's name was changed. I owned a 12/71 built '72 Q code Gran Torino Sport with a 351-CJ label on a chrome air cleaner lid and CJ on all the paperwork. My 5/72 built Q code '72 Sport had 351-4V on a chrome air cleaner lid and other paperwork. The Torino's had the same engines, but a well-orchestrated game was going on with renaming these 351C engines. The '65-72 Ford Master parts catalog (MPC) identified the 351 Q as a CJ. The 73-79 catalog referred to the 351 Q as a 351C 4/B. There were occasional references to the CJ engine in some '73-74 service manuals, but the shop techs knew they were all "CJ" engines regardless of what someone called them. The '71-74 engines used the same style block (machined for 2 or 4 bolt mains), crank, cam, springs, etc., with the '73-74 engines receiving cylinder head, intake, and piston changes to accommodate the new EGR systems.
The manufacturer's desire to move away from any reference to performance had a lot to do with the CJ name disappearing from air cleaner lids, window stickers, and all other printed matter. With gas prices changing daily, economy and safety, not performance, were the new words for the day! The government was always busy finding new ways to spend our money and protecting us from ourselves, and the insurance companies watched the Big Three closely. They wanted to ensure we were offered cars with the same excitement level of a new toaster oven but were slow, safe, and got better fuel mileage than those fuel-guzzling, loud, tire-squealing performance cars we all loved!