Depending on the price of the Magnum engine you're looking at, throwing a carb and intake on that engine may be a simpler/cheaper/faster alternative to rebuilding an on-hand engine. You could sell some of the EFI stuff, too.

The Magnum engines have LOTS of benefits that may entice you - better heads for swirl and quench, roller cam, thinner piston rings for less friction, etc. More hp and better MPG do come hand-in-hand with a 5.2/5.9 Magnum (versus an LA).