318 Magnum swap?
First, decide if you want to do engine AND transmission. To swap the transmission you get into some custom crossmember fabrication and interior tunnel modifications. If you only want to do the engine swap, you can usually find either Ram, Dakota, or Durango engines fairly inexpensive. A lot of donor vehicles will cost less than $1000.
All of the major engine type modifications are well documented. There are some minor details that need to be covered. I think you will have to use aftermarket motor mounts for the /6 crossmember. Someone will correct me if I'm wrong! LOL You may want to upgrade to a stouter rear-end. Mine had a 7 1/4 that I upgraded to a 8 1/4 Trac-Lock. I also had to change out my driveshaft.
Do your research before jumping into this project. Decide how much and how far you want to go into the swap. There's a lot more information on the subject now than when I started about a year ago - Thanks again to FABO.