So now that I've found shorty headers for my 68 with 351w, C4 I'll need to take it to the local muffler shop to complete the exhaust. I already had them install dual exhaust with Magnaflows last year with the stock manifolds so they'll need to modify it for the shorties. I'm considering asking them to put in an h-pipe. They're supposed to increase lower end torque, deepen the exhaust note a bit and practically speaking the can help by stabilizing the whole exhaust system. Any disadvantages you guys know of? Harder to drop the tranny if I need to in the future?

 

When I got my truck, it was already plumbed with an h pipe using the original manifolds from the 460. The mufflers were shot so I had them replaced with the 'cherry bomb' type about halfway back. I still need to replace the pipes from the axle back as they have some pinholes.
I can't comment on if it has more torque with or without the h pipe there, but I think it sounds really good- a low, deep tone that's still mellow and not too loud.

 

I looked into an H for my slick, whenever doing research into things like this my practice is to dredge all the different internet archived forums for opinions. There's over 20 years worth now probably. The only thing I found against them was the claim they make an engine sound like a Mustang 5.0, these types of comments were from old school hot rod folks, where the norm was true dual straight pipes with no crossover. I asked the guy installing the exhaust system what his preference was on his rides and he said no crossover pipe, so I didn't. Can always install one later.

 

If your going to do it a X pipe is the way to go. H pipe is my second option but either way I won't run an exhaust system with out one of these set ups! Helps hp and torque and balances out the exhaust pulse.

Just last week I had a set up installed on my '77 F150 Shortbox 4x4 with a 460 in it. Stock exhaust manifolds that I had jet hot ceramic coated. Pipe size is 2.5" all the way back. Custom bent. Magnaflow mufflers.

Later, shortbox4x4

 

Animation of how exhaust flows through an H-pipe vs an X-pipe, at low exhaust flow velocity and at high exhaust flow velocity.

 

Here's a bit of what I know about the H pipe or Torque Tube.
It started in NASCAR, IIRC, and showed a very good increase in Torque. The are some videos of Richard Petty flipping over on the inside retaining wall and one can see the TT so this started long ago. The factories soon started putting them on production cars. My '62 Galaxie 406 Tripower had a Torque Tube. My '69 Cyclone GT 428CJ had one. And my '69 428 Mustang had one.

I have had one installed on every car that I have owned and there was always a noticeable increase in low-end torque and a nicer quieter exhaust note.
One is supposed to place the TT at the first harmonic for the RPM that you want the engine to be running. IIRC this was 43.5" from the exhaust valve on a typical Ford 390. (Don't quote me as this information has been rattling around in my skull for about 40 years. Most of my cars I didn't worry too much about finding the proper resonance length and I just placed them in the handiest location near the approximate length.
An X pipe is supposedly better but it isn't always an easy thing to do.
For my 428CJ powered truck I am taking a 3" diameter pipe and squashing it down making an oval tube the height of my 2-1/2" exhaust. I'll use my handy dandy hole-saw to cut the sections to fit against the 2-1/2" pipes and then cut holes close to the correct shape. Clean the rust off and weld them in. Then right behind the TT I am going to cut both pipes and put in a slip joint so that the exhaust can be removed if I need to. I'm also putting a fitting made from a sparkplug anti-fouler into the TT so that I can plumb in my wideband O2 sensor.-

 

I don't know the proper industry term or any Ford specific term for what it's called but, if you've ever seen the ball and flare style exhaust connections on a Fox or later Mustang, looks like that would be the ideal setup, since the flare on the end of one tube seals against the ball on the other tube (or header collector) with a bolted flange. There are no gaskets in between to blow out or leak or to have to replace. Simply unbolt the flange, remove the exhaust section, do whatever it is you need to do, put the exhaust back together, anti-seize and tighten the bolts, done.

 

X pipes are supposed to increase power at high speeds, H pipes for low end torque, they increase scavenging of exhaust gasses. H pipes are probably a lot more common with hot rodders or DIYers.

The old school method was to run a line of paint down the exhaust tube. Where it stopped burning off, that was where to install the H pipe. That location isn't always practical depending on the vehicle.

 

Good discussion here. Interesting video. Exhaust is VERY subjective, and after spending a pile of money most are subject to the placebo effect.

Option C is a box for a cross over, aka the MAC power chamber. It appears to be just a box, without baffles (chambered mufflers use baffles). I also added resonators to it.