Getting Ready for the Magnum Swap. (help?)

OH, man there are so many variables in that.
Things like ignition, carburetion and headers have so many possibilities it endless.

There's throttle body injection.
Port fuel injection.
Then straight carburetion for some basic answers on that subject.

Then there's EFI controlled timing. (physically locked out electronically controlled distributors)
Distributors with and without vacuum advance.
Capacitive discharge and HEI ignitions.

If you want to keep it basic and run a standard ignition and carburetion (called naturally aspirated) you can run a standard type electronic (factory style) distributor and some sort of HEI style ignition. or MSD box.
Or even one of the many styles of home made high powered ignitions, like you have probably seen on here that guys have done WAY cheaper than off the shelf branded parts.

A basic 5.9 can be done pretty inexpensively carbed with an electronic distributor.

Make sure to keep or get a compatible flywheel/flex plate to mate the 5.9 to your 518.
The factory 5.9 flex plate works just fine.
A carb in the 600 to 750cfm range works. (600 being a hair on the small side, but gets decent mpg)

You will for sure be doing some floor cutting for that 518, and you need to consider how to you want OD and lockup controlled.
There are aftermarket computers for this but they aint cheap.
An automatic shifting system controlled by the governor in the trans can be built at home a lot cheaper.

Just a ton of options are available, so I would start by deciding the basics first.
Carbed or EFI?
Basic OEM style ignition or spend hundreds on something else?
Electronic or physically (distributor) controlled timing?

Then we might be able to narrow down the choices for you.