360 heads on a 318. good idea?

This has been done since 1971 when the 360 was introduced (using basically a 340 head). There are some pros and cons to the swap.

Pros:
Larger ports and valves have significantly more flow and horsepower potential.
360 heads cost the same or less to rebuild making them an economical upgrade.

Cons:
Larger combustion chambers will reduce compression.
Larger ports may hurt low RPM throttle response.

The loss of compression on the street will more than offset any gain in flow, so add the cost of milling and shorter pushrods or domed pistons. 300 horsepower is about the point with a 318 that the bigger heads make sense, but you can certainly make 300 hp without them. If your cam is less than 230 degrees at .050 lift, you likely won't see much gain.