GM s/b eng. number? Cyl. heads?

Here we go! I've looked at several web sites an find only Part of info. I'm
looking for. My GM eng. # stamp on block in front right bank is f0426dm
As near I've been able to get is Flint Mich. June 26th here where I lost it
1976? and what cid? Been unable to find code DM. Any body??
Is suposed to be mid 57-58 283cid.corvette.LOL also cyl heads unable to find any marking but on front of head is half diamond./\ Was told these are called power pack heads? Any body? Thanks in advance for any info.

Chevrolet used DM two different times....
DM 1959 283 Powerglide, a/c, 230hp, 4bbl carb, full size car
DM 1966 283 A.I.R., Powerglide, 4 bbl carb, Chevelle
As far as the heads, the casting number is on the top of the head, under the valve cover.

After posting this i did alittle more looking.And found casting #
I found it is in fact a 57-62 283 Thanks for the info techinspector.