It can be done fairly cheaply if you pull everything from the junkyard. Don't bother with the magnum v6, it gets no better mileage than the magnum 318. I would run the overdrive trans, it has been proven to be able to fit under a challenger without floor/frame mods, other than a custom trans crossmember, but that's just a bolt-on. You will want a factory service manual for the year/model of donor vehcile so you have a complete diagram of both the wiring harness and the bulkhead connector pinout. Then you move to start stripping out everying except the engine control parts of the wiring harness.

Really I would not recommend the magnum efi swap just because of the lack of aftermarket support for the magnum efi controller, it limits your ability to mod the motor because the stock efi computer just can't compensate for it. I would suggest an aftermarket efi controller for more tunability, but if you're sure stock magnum performance is what you want, go for it.