Lets be careful here. Just so you know this. Keith Black does NOT offer a forged piston. If they are Keith Black, they are Hyperutectic cast pistons, not forged. Icon offers a forged piston through United Engine and Machine. Icon and Keith Black are two completely separate entities.
Now to your question on the heads. If you go to a 340 or 360 style head (same basic difference) you will drastically lower your compression ratio, since the 318 heads have smaller chambers. Plus, most any old iron castings will require a lot of work because they are likely worn out. You will reach out and almost touch aluminum head prices for what you will have in reworking a set of old iron heads.
Lastly, the "X" heads ain't all that. They are basically the same as all the other open chamber smog era castings, but they do have "slightly" better ports and of course the bigger valves. I just wouldn't go the X heads. I'd go aftermarket aluminum. I tell you what I'd do FIRST, though. I'd make DAMN SURE what I HAD was tuned properly and had a good hot ignition curve, initial and total timing set up right and make sure the camshaft was properly degreed and everything else was tuned properly. That in itself can net you some big power gains if it is not properly tuned. If the heads are ported as you say, and you have some good pistons with raised compression, you should have a pretty spanky 318. You "might" already have the best option and maybe it just needs some tuning.
But I reiterate, I would NOT put the 340/360 style heads on it, UNLESS it was a closed chamber aftermarket aluminum head.