1968 Gto

[nevada]

I recently aquired a 68 GTO with a 400 engine. When I step on it I get a rattle for just a second. I am running premimum unleaded fuel with a 10 point octane booster additive. I was told by the seller it had hardened valves to run pump gas. Any advise would be appreciated.

[MrPbody]

Nevada, what heads are on the 400? Originals on a '68 would be "16s". You can find the number on the outside of the center exhaust ports. Those heads have 72 (nominal) CC chambers.
It sounds like you have detonation. If that's the case, you can try backing the timing up to give you no more than 28 total, but it will take all the power away, and may raise operating temperature to an unbearable point.
The only REAL fix is to remove the heads and drop the compression to below a "true" 9.5:1. Pontiac heads have very efficient combustion chambers, which make for very good runnners IF all things are correct. With today's gas, 93 octane just isn't enough to supprt the 10.5:1 advertised (about 10.2:1 actual) compression ratio of the muscle car era.
Changing the heads to later ones with larger chambers is one way to go. Another is to remove the pistons and have a "dish" cut into them, providing more chamber volume.
Beware of those claiming to have outsmarted the laws of physics. It's true, you can push the edge by meticulous tuning, but the gains are hardly worth the effort. Lowering the compression is the logical and permanent fix.