I have done a couple magnum swaps.
Probably cheaper than most.
I buy a truck. Rusted rotted junk for 300 to 800 bucks. Pull the engine, flex plate, starter, and anything else worth saving.
Sell any parts that you can sell of the truck quick and easy.
Scrap the rest for most of your money back.
Buy a van power steering pump bracket,use the same headers or manifolds from your 318. Buy a Chinese air gap intake,use your 318 distributor and the magnum ignition coil.
The last one I did was a truck so I used the truck pan but in the dart I used a cheap ebay car oil pan.
On the truck I used a 5.5 psi max electric fuel pump and no regulator. I used the fuel filter with the vapor line in it and I figured it would bleed off any excess fuel pressure.
Reground cam from Oregon cams and cheap pushrods from Summit make a fun little street car.
I keep harping on doing magnum swaps this way because it is just so damn cheap to make a massive power increase over an LA.