I have a 1972 camaro with a 400 bored 30 over to make it a 406. Its was built at 9.3:1 compression and has a large hydraulic cam. Its backed by a t56.

Its happened a few times since I got the car and is happening more frequently now, when I turn the car off it keeps wanting to run for a few seconds and clunks until it dies. I read that sometimes its from carbon buildup increasing compression so I ran some seafoam through the car and put a can in the gas tank with 91 octane.

Its still happening, anyone have any ideas or more information on what causes this to happen?

First make sure your idle isn't too high. A too fast idle leaves the primaries partially open and causes dieselling.
If that doens't solve it, make sure you have high enough octane gas in it and try advancing the timing a little. When you do this, the idle will increase in speed. Turn the idle back down and drive it. That also might solve your dieselling because it closes down the primaries on the carb.
Like you were already told, carbon buildup on the pistons causes this too, so if you aren't convinced there might be other problems, try cleaning the combustion chamber again with GM Top Engine Cleaner. It is what the factory recommends and will probably do a better job than Seafoam.

I'm having the same issue. Tore it down last week and cleaned the carbon out.
91 octane, 14* initial timing, richend the idle, put in 5* start retard, going to try colder plugs, NGK '5's next.

I find it hard to believe its carbon buildup, the engine rebuild only has about 1000 miles on it. I do have my idle to high, at about 1000, but I cant really lower it. With the A/C on and the huge cam it needs to be about there or it dies on me when I take off. I'm sure its a case of me having no knowledge of a carb what so ever. I'll have to look into that and see what I can do.