economic 318 aftermarket heads

Not all 318's from that era had 302 heads, there were a couple of models.

Also, LA 318 heads have smaller valves, smaller chambers, and smaller ports than LA 360 heads. Just swapping on an LA 360 head will drop compression on a 318, which is already low. LA 318 heads are usually 57-62cc's, 360's can be 65 all the way up to 72cc's. The 302's would be a good choice if you can find a set. Chambers are usually around 62cc's so there's usually no change in compression, possibly a small drop if you go from the smallest 318 chamber to the largest 302 chamber. Plus they'll have hardened seats already.