340X heads
find a set of 360 heads (the x heads might work im not sure) and get them plained down to get the compression down to where you need it! these heads will flow a lot better than 318 heads! with plained heads a nice cam, intake and headers that car will move!!!
All 340-360 heads are very similar and theres near zero difference between a J and X head. Save the intake valve in some J's.
While farmer260 is correct in the 360 head flowing more, it is in how it's done and whatit is on that will prove to be a power maker or turd. I have to disagree here on a 340 - 360 head on a mild 318. The port volume is huge. This makes the air and fuel slow moving and un-vaporized, especially at low speed. You'll need some really high RPM to get the heads to work well for you. And it is not in the driving range of the cam, your choosing.
The 318 heads with some work done will provided a higher velocity air and fuel movment and a finer and more fully vaporized air and fuel charge for a better burn in the cylinder. This IS power!
For a on the cheap improvement, (340 - 360 heads) it is OK, but if you have a few bucks to spend, spend on upping the anti on the 318 head or do a Magnum head swap.
Like Adam said, the 318's "302" head should be sought out. But even some basic port work on a late 70's or 80's smog head will show a nice improvment.
(IMO go with a 1.88 intake valve. Stainless steel if you can. Like a Ferra )