i wouldn't focus on a 68 date code, or any date code manifold for that matter. you'll pay a premium for what any of the others can provide at a fraction of the cost. also, the 318 4bbl manifolds (and some 360) match the smaller 318 heads port size-- not that that's a huge deal.
your stock air cleaner won't fit, at least not with that base, but pretty much any of the other factory 4bbl ones will. they repop the early unsilenced ones that look the part and they aren't crazy expensive.
piston & cam choice would depend on what you want as a performance end goal. and how much machine work you want to do on the 318. for what it sounds like you're trying to accomplish there's nothing wrong with stock replacement cast pistons and a 360 2bbl cam (slight bump up from a 318 cam) if you want to keep it dead simple.
i just put together a 74 318 with stock 40 over pistons, 68 heads, 340 cam, LD4B intake and a small 4bbl with manifolds and it straight rips. there's no magic there, just careful selection of parts that all work well together. while i wouldn't suggest this is something to copy exactly as the rest of my combo is entirely different (4spd, lighter car, deep gears), it's something that kind of lays down the blue print of what can be done at reasonable cost with decent components and get a satisfactory result.