273/318 Swap

The mounts, oil pan & pickup are a direct swap to the 318. If you get a later model 318 (~'74 and up?) you may have some clearance issues with the driver's side exhaust manifold at the #7 exhaust port. The oversized mounting pad for emissions (air injection ports, all of which will need to be plugged, BTW) can cause some conflict with the manifold "downleg". A bit of grinding (don't get carried away on this, you'll be sorry) or an extra thick exhaust gasket; or combination of the two may be necessary. Earlier teens don't have this issue.
Passenger side is no problem, but you may need the 90* oil filter adapter from the 273 to clear it if your 318 doesn't already have one.
Make sure your crank is drilled for a pilot bushing, or you can use a later Dakota roller bearing after making sure the relief is deep enough.