I have read this before as well, that the factory engineers calibrated the magnums rich enough at full throttle to be able to run without any exhaust system and still not lean out. Anything you can do to help flow is beneficial here. I have a digital copy of the MP porting templates for magnum if that's something you want to pursue you can PM me.

I left my conversion basically stock, however everything I have read indicates the stock ECU, which is not able to be reprogrammed by the way, won't tolerate much in the way of mods. I would focus on cleaning up the heads, tightening quench and freeing up the exhaust, although if you don't plan on pulling the heads I'd leave it as-is.

I don't think the stock ECU would like the aftermarket M1 efi intake, but I have no practical experience with it. I can tell you the stock intake is known leak oil into the intake tract, hughes sells a kit to remedy this.

If you drill out the holes on your crank sensor, you can slot it and move it sideways, this will gain you a bit of ignition timing as playing with the distributor won't do it on the magnum engines.