Putting a 318 mag in 74 duster

Nothing. Nada. The main hookups are all the same, slant 6 or V8.
The only changes that may be necessary are regarding wire length- you may want to shorten some of the leads to the distributor and oil pressure sender, or just loop them up and tape them; your call. You MAY need a longer starter cable, but that's it.
As long as you're using factory components (alternator, ignition, etc.) you shouldn't have to modify a thing.
Don't forget about bypassing that pain in the a** seatbelt interlock on the '74s. Lots of threads on that.