H-pipe, good
Here's my $0.02 on the H-pipe:
I've read a lot about straight dual exhaust vs. H-pipe vs. X-pipe, and the general consensus seems to be that the H- or X-pipes will give a performance benefit over straight duals on any engine. The reasoning I've heard is that a connection between the sides allows "evening out" of potential pressure differentials between the two sides due to the timing of exhaust pulses. This allows the exhaust gases to flow more quickly and uniformly through the exhaust system (this is my understanding of it, anyway). Seems a little like voodoo to me, but peformace tuners seem to swear by H- and X-pipes.
Having said that, I would tell you that my experience with an H-pipe is limited to the one that I installed on my wife's '02 'Stang. We got rid of the stock exhaust system with its four (yep - count 'em, FOUR!) catalytic converters and installed long tube headers, a catted H-pipe setup, and a new cat-back, and it totally transformed both the performance and (of course) the sound of the car. I'm in the process of rebuilding a '67 Cougar right now, and I fully plan to put an H-pipe set up on it as well (if I ever get that far). But I suspect that any such installation would have to be a custom job on these old cars. I haven't looked for such parts for our older cars, there are a LOT of parts available for the later model years (late 80's up), and I suspect that those parts are what saturate the market. However - I would think any reputable exhaust shop would be able to weld you a set-up without too much trouble.
Hope this helps - good luck!
Lab Rat