Best 318 headers

Rumblefish covered it pretty well. To just add a little to his comments; in terms of fit, you have more off the shelf choices that should fit than an A body.

As far as an H-pipe goes, it can do a couple things. One, if flow gets really stuffed up, it can help share the volume with the other side. This is not going to be a problem in your case unless you have very poor flowing mufflers for some reason. Two, it can help with the harmonics which in turn can help reduce sound if placed in the proper location. Additionally, if the location is about right, the pressure waves formed will help flow in an rpm range that you may care about.
Pipemax is a relatively low cost program that can ballpark H and X pipe locations if you know enough about the engine. I've not run a 318, but have run a number of variations of a 340 (different cams, compression ratio, VE etc). Based on that I'm going to guess that ballpark an H for your setup should be 19 to 24" from the end of the primary tubes. Don't expect a lot, but at least it won't hurt having it in approximately correct location if you do it.

IMO your biggest problem with headers will be that most were designed for 340/360 applications. Used on a 273/ 318 there is a big step up from the head port to the header. The step, especially if noticible at the top, will produce turbulence and the effect is somewhat unpredictable. Additionally, once the exaust gas gets into the primary tube the gas slows down because the tubes cross section is much bigger than the port. The faster the exhaust gets sent away from the head the less that can be sucked back in during valve overlap. Ideally a 273 or 318 would exhaust into a 1.5" primary, then possibly step up. Not many off the shelf 1.5" tube headers - but if you see one, its worth looking into fit etc. IIRC Sanderson may offer one.

There's a number of exhaust theories out there. The ones we know about that I respect the most come from Larry Meaux, engine builder, tester and creator of Pipmax softeware; and Calvin Elston, exhaust builder. I've relied both as appropriate for my current header project.
Some of Calvin's thoughts on tube sizing are here, which will explain in more detail my comments above:
Exhausting 101