The 427 had a 4.23 bore and a 3.78 stroke. The 428 a 4.13 bore and 3.98 stroke. The 427 was Fords all out engine and was expensive to machine and build. Plus warranty problems being most were raced. They looked for a cheaper alternative and still have a good performing engine. This is where the 428 came into play. The 427's bottom end was crossbolted and 65 and later (except 68) used forged steel cranks and rods. These were expensive to use plus the blocks much more time consuming to machine. As we know time is money. Ford wanted a cheaper performance engine for street use. The 428 was the choice and first came out in 66. The larger stroke gave boatloads of torque with HP at an average 345-360 in stock trim and close to 400 in Cobra Jet and Super Cobra jet applications. The smaller bore 428 blocks were easier to cast and machine plus they were all 2 bolt main blocks unlike the crossbolted main 427. Plus the 428 was about half the cost of the 427 to produce. Ford first used the 4.13 bore in the 406 engine. The 406 had a 4.13 bore and 3.78 stroke. The 406 was the precurser to the 427. Sort of a tweener if you will and only produced for 1 year 62 1/2 to 63 1/2. Listed at 385 hp with a 1x4 and 405 with a 3x2. If you want to do some 428 reading check out www.428cobrajet.org By far the best 428 site online. I could go on but will spare you my endless babble. LOL.
G.