I have a 68 428. Without going into the motor and looking at the bore, stroke or crank serial it will be hard to tell for sure. By chance is there a welded "A" or "C" on the back of the block? Not all 428's had them but it may help in identifing the block. Also check under the front of the drivers side head for a flat spot where the partial VIN would be. They started putting VINs on the block in late 68. You may get some motor info from it.
--Mike