1973 Dart Magnum Swap

Can I use the stock harness and PCM from the ram?
Yes, if the Ram doesn't have a security PCM, if it does, you'll need to send it out to someplace to get it flashed to remove the security feature ( guy out of Lantana FL does it )

What about relays/fuses?
Yes, The wiring will be a lot easier if you keep the Power Distribution Center from the engine bay. It'll have all the wiring for the fuel pump to injector drivers and coil.

Do i use the drive shaft from the ram?
Depends on how long it is. Any driveline shop will be able to shorten it for you and put the right rear yoke on it.

What mods for the mounts?
Shumacher mounts or build your own. Making your own requires more time than it's worth ( that's what I did, wish I just bought the mounts )
...wait, if you've already got a v8, just shim up your stock mounts and use them. Pretty sure the ears on the block are wider on the 5.9, not sure though.

I know the crossbar for the trans has to be modded, what about linkage?
If your car is an auto now, the old linkage should work. Just use the arm that came on your 727/904, remember the 4?RE/RH is based off the 727 and the 5??/999 is based on the 904. The kickdown uses a cable from the throttle body so that should be good to go, just make sure it's adjusted correctly. The books call it a TV cable ( throttle valve )

Will the fuel pump from the ram work?
Nope, unless you're planning on grafting the part of the fuel tank into yours, not for the faint of heart.
Some of us use the corvette fuel filter / regulator with return or find the earlier magnum fuel rail with built in regulator.
I use a generic Airtex pump after the rust from the tank killed my walbro.

Congrats on the swap, you'll love the fuel injection, I know I do.