I think barracudadave means it's not a good idea to put the HEI Module in the box, although he said ballast. I have the HEI on an aluminum plate now I don't see how the hollowed out Mopar ECU box will hurt anything....
The HEI modules are inside a 200 degree distributor normally anyway, but as long as they have something to dissipate the heat to through conduction it should be fine.
