Just a shot in the dark here...
While I don't have this problem in my truck, I've always had it in my air-cooled VW. When the engine is shut off, the carburetor begins to heat soak. After a few minutes the gas will start to boil and "dribble" and collect on the throttle causing a rich restart.
I have to hold the accelerator to the floor while cranking to "pass the gas"!
I've never seen this happen in my truck, but if your fuel level is too high in the float I suppose this
could happen. To check for this, once the engine is warmed up quite toasty, shut it off, and watch down the throttle to make sure no gas dribbled down the onto the throttles. Good luck, and please share what you find!