Quench

When you built the engine, did you check piston/deck clearance? if your pistons are .020 in the hole, with a .040 head gasket your piston to head clearance may be in .060 range, explaining your detonation problem. You want .035 to .045 for the quench effect to quell detonation.

That's a pretty short duration cam to be running that high of a compression ratio in combination with tall gears, high weight and large tires.

i wouldn't worry about over scavenging either with the longer exhaust duration. It just allows you to run a quieter muffler. Magnum head exhaust flow isn't that great to start with either.

If you plan on increasing the comp ratio, you really need to add a bigger cam, whether you drop this in the Dart or not.

It's all about the combo.