74 duster slant 6 to 318 conversion

Kinda vague, but I'll try:
Assuming the slant was an automatic (904), all the shift linkages and driveshaft should bolt right on. You should even be able to tweak the trans cooler lines to fit.
Assuming the motor came out of a passenger car and not a truck/van, the oil pan should be okay.
Assuming the motor came out of an A body, so you'll have the correct kickdown linkage to clear the A body firewall.
Assuming the engine came with accessories (pulleys, p.s. pump and brackets, alternator and/or brackets, and whatever AC brackets/compressor you may or may not need), you should be okay there, too.
Some of the wiring harness connection points may be too long or too short for the new engine, so be prepared to do a little work there.
Assuming the 318 is a later one with electronic ignition, that should all plug and play.
Assuming that your 318 isn't one with the Lean Burn ignition, otherwise you'll have to swap that out- distributor, carb, and a few other miscellaneous details.
If the 318 came with exhaust manifolds, make sure that they're A body manifolds.
Mainly, you will need an appropriate A body V8 radiator and shroud, and POSSIBLY a shorter V8 throttle cable.
An exhaust system.
Possibly some other details I'm forgetting, but that's the gist of it.
It would be handy to know the year/original application of the 318...