Obviously, on a FE Forum, the FE would likely be the majority choice, a Cleveland site may well show different thoughts. Regarding engine compartment fit, and spark plug access, my first 2 Mustangs were M code 70s with 351C 4 barrels, and I have also owned 3 428 CJ 69-70 Mustangs, 2 428CJ 69 Cobras, and a 66 Fairlane 390 GTA. What I feel, is that although the 351 C is (slightly) narrower, the spark plugs are buried inside the heads a fair bit, inboard of the exhaust port bosses. So changing the plugs on a 351C is more by feel, than visual, especially with the thick factory exhaust manifolds, even more so if the factory lifting brackets are still in place. Unless running tall valve covers, like the "Cobra LeMans" style, I never found the plugs on the FE all that bad, although I must admit that none of my cars still had the Thermactor air pump and plumbing, and only 1, one of the Cobras had A/C. And if the FE does have the tall valve covers, if you remove the covers, there is plenty of space to get to the plugs. I would rather do the plugs on the FE than the 351C.