Cost of a swap

It all depends on your experience and resources.
I'll cover a couple of the big ones you will be up against depending on how you would like to do it.
You can do all Magnum style with EFI and the serpentine belt system, or carbureted with LA motor Timing cover, water pump, pulleys and intake with aftermarket heads with LA pattern intake bolts like I am doing.

1. Almost all running Magnum heads are cracked or going to crack.
You can get by with cracked ones some of the time, but I hesitate to say most of the time.
(There are a lot 5.9's with cracked heads out there that run just fine)

2. If you plan on running the serpentine belt system and don't get it all with the engine you will have to come up with it somewhere.

3. You either have to get the 5.9 balanced flywheel or buy one, or use a balance converter if you plan to use your existing trans.

4. You will need to deal with the drivers side motor mount being a bit different (It can work with a few spacer washers) or you need new mounts.

5. I you plan on running EFI then you will need the entire fuel and computer system to run it including the high pressure "in tank pump" with built in regulator.
Otherwise you will have to buy an aftermarket EFI controller system.

6. Most cooling systems that work fine for a 318 may not for a 5.9 (they produce more power and therefor more heat.
Those are about the biggest and if you can't do any of it, the most costly items to deal with.

I am in the installation stages of a performance 5.9 and A500 overdrive (42RH) swap right now, but I'm going with the LA style carburetion and belt system so if you have specific questions of going either way I can help if needed.