All Mustangs until the '71s were based on the Falcon platform. This is one reason Ford made two very different big block engines in '68 and '69. The old FE big block (introduced in '58 as a 332, later it went to 352, 390, 406, 410, 427 and 428 cubic inch capacities) would barely fit in a Falcon based Mustang, but starting in '67 Ford felt it was important to offer it in 390 and 428 cu. in. sizes. Not many people bought a Mustang or Cougar with a big block, but there were a few, and of course the Shelby KR 500 with the 428. Even changing the plugs on these is pretty hard due to the small engine bay. The 429 and 460 big block (initially the 429 was Ford and Mercury only, with the 460 reserved for Lincoln) is a physically larger engine and just wouldn't fit. So Ford kept the old FE around until the new much larger Mustang and Cougar debuted for '71; these used a lot of Torino bits and the 429 dropped in just fine.
The only exception to this is the '69 and '70 Boss 429. That is a very special car designed to use the hemi-head Boss 429 engine. All Ford wanted to do was homologate it for NASCAR, but the France family that controlled NASCAR was already reeling from accusations that the cars were going too fast for the tracks and refused to allow it. Ford made about 3000 Boss 429s, which had even wider heads than the regular 429! Somewhere I've seen a photo of the engine installation line for these cars, each car was under an overhead crane with a Boss engine ready to drop in, and lots of very clean men in white lab coats were gently persuading the engines into the cars by hand. No wonder the Boss 429 cost a bundle! No doubt any Ford engineer who worked on the original Falcon project was amazed at how much that poor little lightweight platform was stretched and pulled over the years, but it surely attests to the overall solidity of the design.
