Non-crossover exhausts tend to sound "raspy" and uneven. H-pipe or X-pipe smooths things out, balancing the exhaust pulses between banks.
I went from an H to an X, and there is quite a difference in tone & smoothness. It sounds really wicked from about 4000 rpm up, as it completely changes pitch [cold chills] ...
Below is an excerpt from an article I saved awhile back. It's not my writing, but I don't recall the source.......
H- and X-tubes for dual exhaust--
H-tubes balance the backpressure between the two sides of your engine, and even out the complex pressure waves that are flying around in the pipes. The result is enhanced low-end torque, better overall performance, and better sound.
With a V8, there are always two cylinders on each side of the engine that fire within 90 degrees of each other. On a small-block Chevy, the firing order is 1-8-4-3-6-5-7-2, and the cylinders are numbered from front to back down the crankshaft. Odd numbers on one side and even numbers on the other. A cylinder fires every 45 degrees of crankshaft rotation.
So by looking at the firing order, you can see that cylinders 4 and 8 fire 45 degrees apart on one side. Then later, cylinders 5 an 7 fire 45 degrees apart.
This creates a high-pressure pulse that inhibits smooth flow and makes for a "choppy" exhaust sound. H-tubes and X-pipes equalize this pressure pulse, smoothing out exhaust flow and sound, thus helping your torque output. X-pipes do a better job of this than H-pipes, because there is no distance separating the pipes with an X. But some cars, like my 'Bird, have no way to fit an X underneath.
H-tubes should be at least 3/4 the size of your exhaust pipes, and located in the "hot" area of the pipes behind the header outlets. To find that hot area, one suggestion is to spray a line of ordinary white paint down the pipes after the headers, and go for a good run. Install the H-pipe somewhere in the area where the paint burns off the pipes, equal distance from each header collector. Use the same technique to determine where to place an X-pipe.