Change the cam, springs, retainers, and intake and you're done.

Here's the evolution of those two engine programs:

300 and 380 initially came with a stock long block right from the Saltillo Mexico engine line. Both had stock magnum heads. 380 got the cam, springs and retainers changed....300 did not.

Then MP ran Chrysler completely out of OE magnum heads, the OE supplier wasn't interested in making them anymore, and it probably would have cost 500k or more to modify the tools to fit a different foundry line.....So, enter the R/T heads and plan B:

320 and 390 had R/T heads and started out with stock Saltillo shortblocks.

That worked fine for awhile, but it wasn't long before MP ran Chrysler completely out of new OE shortblocks...and it was time for plan C:

They both got .020 over reman shortblocks with FM hypereutectic pistons, R/T heads, and otherwise the same specs as before.

380 had a stock piston.

390 in it's final, .020 over form had a better piston with a couple vavle reliefs.

390 in it's very first form had a stock piston.