318 into /6 '74 valiant sedan

Engine mounts,brackets and a V-8 K-member is the biggest thing. Schumacher has adapter mounts to put that teener on the slanty K-member.
If the engines both have the same size transmissions, then the driveshaft should be fine. If the engine has no trans/convertor, well then, start hunting.
You will need a new exhaust system,duals recommended.
The 7.25 rear could be fine for a few decades more,so long as you don't peg-leg it.
A 318 will survive on the /6 rad, with a direct drive factory type steel fan of 5 blades or more.
The 74 should already have front disc brakes.
You will need the brackets for various things like P/S, the alternator, and A/C if she gots it, and transmission linkages and a V8 throttle cable.

This is about as easy as swap as there is, and I have done several.

Easy swap/easy money. Happy HotRodding