380 hp Magnum crate dieseling problem

That crate motor has the 288/292/108 cam in it right? It likes a lotta idle air bypass. You are probably up the transfers too far, and when you shut her off the flywheel momentum continues to spin the crank. This pulls in air and fuel past the too far open throttle blades, and that mixture is exactly the same as it was before you killed the ignition. So just the heat of compression can cause the mixture to ignite.
The cure in my case was to close the secondaries up tight but not sticking, then set the T-port sync to a little taller than wide, open up the mixture screws to 3/4 turn,make sure the PCV is working,(you are running a PCV right?) and the Vcan is on the sparkport, and I set my timing to 14* with a 4 speed, otherwise that cam gets jumpy with more. Then you have to give it bypass air somewhere, else the idle speed will be too slow, and stalling on take-off becomes an issue. My solution was to drill holes in the primary throttle blades, on the Transfer port side about half way between the shaft and the edge. The tricky part is to choose the right hole-size. So I started small, and snuck up on it. If you go too big don't panic, you can solder up the hole, move over, and start again.
Start with one hole in each primary blade at 1/16. I know 1/8th in each will be too big. I think I ended at 3/32s; your results may vary.
If this scares you; you can Tee into the PCV line with one of those 5port tees, and use Holley jets in small hoses, to determine what your engine likes. If you spend a lotta time at below 2000 rpm, and you find it jumpy down closer to idle, you can soften the pressure pulses by running less timing; I run 14* with a 3.09x3.55=10.97 starter gear. It will idle down to 550 in low-gear at 5* advance. If you have been running a Vcan on manifold vacuum, you will have to give that up; put the can back on the sparkport.
Then go back and fix your power timing, and rate of advance, and tune in the Vcan as early, and as fast, and as much, as you can get, I run 22* in that can.