[patrickt]

Originally Posted by Fordzilla

Steve, I think the true 428 service blocks had that casting.

Let me just rip a quote and pic from the 428 Cobra Jet site http://www.428cobrajet.org/id-block.html

Some blocks have a "428" cast onto the floor inside the water jacket. There are two common positions to view this protruding number: one is just inside the center freeze plug hole, the other is directly beneath the long coolant slot at the rear of the block. If present, you can be sure the cylinders are thick enough to support 428 pistons, core shift permitting. If the number is not visible from either position, you'll need to make other verification checks. Measuring the gap between cylinders using a drill bit is the easiest way to determine whether your block is capable of supporting 428-sized cylinders. It is presently thought that if the shank of a 15/64" drill bit will NOT fit between the cylinders at any reachable spot in the six core holes, the cylinder will likely handle a 428 bore, core shift permitting.



"428" Casting Mark Inside Water Jacket
Picture courtesy of Paul Cavaghan