455 diesels when I shut it off first time it ever did it put 87 in it just to test new transmission been run premium otherwise so I'm guessing it must be a higher compression motor 396021F with E heads any info you guys got thanks

 

If your idle speed is too high it will do this.

 

As mentioned idle speed can do it, also go back to 93 octane ASAP, and even octane boost. In the short term you could leave it in drive and turn ignition off.

 

Dieseling at shutoff is nearly always an idle speed issue. Would be worthwhile to verify idle ignition timing and cooling system effectiveness, too. Too little spark advance (broken vacuum advance, for example) leads to higher than optimum temperature, which makes dieseling more likely.

 

Since it did not diesel until using 87 and that 87 octane is too low for an engine assumed to be a 1970 motor due to the E heads, my bet is on low octane being the source of the dieseling. The 87 will also ruin pistons in short order. Do not push the engine hard and get better fuel in it ASAP.

Good luck!!!

 

Insufficient cooling/overheated combustion chamber leads to the ignition of the remaining fuel after key-off (no spark). Could be you have been operating on the verge of post-ignition for some time and the higher octane fuel is just enough to mask a problem. Obviously, run the highest octane you can get but also look into the possibility of something causing higher combustion chamber temperatures.
You don't mention anything about your exhaust system but if you have the original exhaust manifolds, it's possible (probable) the spring operated valve in the driver's side manifold which is intended to help with faster engine warm-up has a rusted/frozen shaft. This is pretty common. If that valve is frozen in the semi-shut position it'll restrict exhaust flow on that bank and drive your chamber temps up.