If you use the motor, trans, wiring and computer out of your donor truck it will work, but you will need to attack a few issues.

The motor will drop right in just fine and bolt up you old 318 motor mounts. The 518 should fit but you will likely have to take your angle grinder and grind down a few of the reinforcing ribs on the outside of the case to get it to fit. Another member came up with this and it seems to work great and do no harm. You need to do this because the 518 is fatter than the 904 or 727 it's replacing.

You will have to custom weld your transmission mounting bracket. You can take your old bracket and cut the mounting ears off it and throw away the rest. The middle part of it will have to be custom fabbed by you. No way to get around this part. Other than that the mechanical install is not too difficult. Driveshaft will need to be shortened, you can measure for this after you get the trans installed.

Fuel system will need to be upgraded to a high-pressure efi pump and have an efi pressure regulator and return line. You can use a walbro gsl-392 universal in-line pump if you don't want to custom fab your own internal pump. Still not as good as original because your carb tank doesn't have a sump, although you can get by without it.

Then you get down to the wiring. It's not overly difficult, just very time consuming and requires you to gain an understanding of the mopar truck efi system. When I did a conversion similar like this I at least had a factory service manual pinout diagram of the wiring harness and a diagram showing all the engine control functions of the wiring harness. Basically you need to disect the wiring harness and just keep the sections of the harness that control the engine and junk the other stuff like the wiring for the trucks headlights, turn signals, etc. Like I said, not overly tough, just time consuming.

Going to carb presents it's own challenges but they are much easier in comparisin. You can buy the edelbrock rpm air gap intake which will bolt to the magnum heads, and need to run an electric fuel pump because the magnum's cam doesn't have provisions to run a mechanical fuel pump. Then you need to rig up either switches or a few sensors to control the overdrive and/or lockup converter on the transmission, however those mods are well documented and instructions are out there.

No doubt going carb would be easier, but if you want the driveability of efi, it certainly can be done. Main drawback of going with the factory efi is lack of tunability. You can only do so much for mods because there's no real way to recalibrate the stock ecu in those trucks as far as I know.