[bmalone]

Mike, there are a lot of similarity between 390 and 428 part numbers. You already have the date code, so you know it is a '68. Block numbers for a '68 428 can be C8ME. Block numbers for a '68 390 C8AE-x, where x=change code such as A, C, or E.

Heads for the 428 CJ are C8OE-H, or C8OE-N for both CJ and SCJ. Standard 428s can be C8AE-F. But notice that '68 390 heads can have these numbers: C8OE-A, C8OE-B, C8OE-F, or C8AE-A. The change code is really the only difference and sometimes the numbers are hard to read. I think it might also be possible that the 390 and 428 share the C8AZ number with the 428 using later change code numbers.

The best way to be sure is measure the bore: 4.13 for the 428 and 4.05 for the 390.