how can i beef up my 318 performance

Above, I see great minds think alike! Both posted at the same time! LOL!

Depending on how far you want to go, the first step is to make a plan and stick with it.
Moving to a 360 is a catch 22 since the money you could spend on a hop up or mild performance gains would go to getting and then machining the 360. (If it needed the work) It can be a good chunk of coin depending on the parts used and what your target goal is. Still, a junk yard 360 plus the cost of a B&M corrective balance flex plate and then maybe engine mount(s)?

As a general guide line;
(This is a general list and sometimes there not so easy to do, but here we go!)

Stage 1, bolt on parts
(From top to bottom)

Upgraded air cleaner, stock 4bbl. or open
Carb to a 4bbl., Holley, Edelbrock or TQ (TQ must use the proper intake or adapter!)
Intake manifold to a 4bbl. General to mile performance Holley or Edelbrock should be 600-650 cfm's.
Headers into a 2-1/4 exhaust (mild) or 2-1/2 exhaust (good for power!)
An "H" or "X" pipe would be a plus in the system. Summit racing or Jegs has inexpensive header back kits with and without mufflers.
It is my opinion that the headers and the exhaust kit be purchased at the same time and installed at the same time as well. The X or H will require some time to splice in, so, take a weekend and perhaps have a buddy around.

Upgrade the ignition, distributor, wires, and ignition box. For a basic plug and play, the MoPar Chrome box or a FBO unit is excellent.

Stage 2, bolt in parts and machine work

A cam is an arguable part for stage 2 since often it is best to do a cam change when the intake is off. The vary-ing cam profiles must work as a package with the rest of the engine and drive train. When you purchase you cam, save money up for the whole kit. (If available)
The basic kit is cam and lifters. But I would option for the timing chain, valve springs as well as the retainers and valve locks etc...

Heads;

Stock heads can be great performers. Putting money into a stock head with larger valves and/or porting can yield great results but you will have to know if the castings final stage of work is worth the price vs. aluminum heads. (Ported or not.)

A stock small valve 318 head can motivate a A body into the 14's, no problem.

Going to a stage 3 is rebuilding the engine for more serious gains. If you want to push your car into the 12's and faster, it is going to more than likely need a rebuild with a few upgraded parts to insure long life and not a short putting bending or braking parts. While possible to run 12's on a stock short block (and heads if it has to be that way) I suggest not to do this. The engines are old with probably unknown history.

Also at this point, it becomes an expense that a 360 is probably the better bet for ease of motorvaition. (Sp?) less cam, not such a snotty rough idle engine, more torque and HO delivered quicker in the rpm range.

To do a 318 at this level, you pretty much have to want to do a 318 that way.