I believe the H comes from the vin#.
The 383 cid engine in -67 A-bodies were limited to 280 hp due to the restriction in the exhaust manifolds.
Been trying to figure out what makes the -68 engine develope 20 hp more then the -67 engine. As the comp. is the same, the cam has a slightly difference in the duration but the same lift.
Maybe it´s the bigger exhaust valves 1.74 vs. 1.60
Did the -68 H code cars use different exhaust manifolds compared to the -67?
The -68 also had bigger combustion chambers in the heads,
guess that it has to do with the "906" vs "915" heads?