If it's a 326 4 bbl, with small chamber 326 4 bbl heads, mix 110 octane 50-50 with 93 and drive with vacuum advance connected at cruising speeds for several miles, then recheck plugs.
If you see an improvement in rattle and plug condition, you need higher octane. My '69 400 is the same.
If you still have rattle but clean plugs, check your distributor advance curve.
If you still have rattle and dirty plugs, check your carburetor's cruise metering and overall condition.
If you have 2bbl heads or other low compression heads, you shouldn't rattle.
Vikki 1969 Goldenrod Yellow / black 400 convertible numbers matching
