[Harley]

@Connor94 just redid the system on his Blazer and it looks very nice. He tucked the cats up in the downpipe section coming off the manifolds, but I don't think this is as doable for you with the headers.

http://instagr.am/p/CnehqvpOZiP/

I think you're on the right path with the single out. It gets really crazy going for the traditional duals - one on each side. Could take a play from the 2000s era GTO and run two pipes on one side, but I believe a single 3 or 3.5 will be easier to package. The 3.5 could be very tight over the axle, though. I merged 2.5" into 3" without cutting any of the factory frame/crossmembers. I haven't worked out a tailpipe yet, so take that with a grain of salt.

I'm not currently running cats, but plenty of room for them along the transmission on my setup. I'll be adding some soon for visual inspection where I recently moved.

What transmission are you running? It will make a difference in how much space is available. My T56 and lower engine height pushed me pretty far back on the crossover, but I think there would still be room for an X pipe even if it's above the crossmember behind the trans.

There is room for 2 mufflers under there. Even if you went with an oval rather than a round muffler they could be turned and set next to each other. If I'm remembering correctly @greencactus3 did this on his ZR2 Blazer build.

With all that said, I'm not sure where the resonator will fit behind the mufflers. Specs show it will end up being 10-13" long and there just isn't a straight piece of the tailpipe anywhere near that long. On @Rhotpursuit pursuit S10 we started out with one muffler, but he ended up getting 2 packaged in there and is much happier with the sound/drone. If it were my truck with the same goals I'd be trying to pack as much muffler x2 in there as I could.

 

[Harley]

Transmission = Level 5 - 4L65e (with Sonnax Housing and overrun clutch hub) from Finish Line. Should be ready any day now. I stayed with the 4L series due to simplicity and space concerns.

There should be plenty of space to get around the transmission and have room for an X pipe. My experience with a 4L60 was on an S10, but it felt much roomier than my T56.

My plan is to run 2 of the Magnaflow 14419 mufflers in the configuration at the top of the thread. These are 3" in/out and only 4" around and can easily be used side by side. Even welded together into a single unit if need be. We'll see if there is any space to add an x-pipe.

There will be plenty of room for these mufflers. I made the suggestion to go bigger in case they're not enough without a resonator. There's tons of up/down, side/side, front/back where the mufflers will go. Not much space if any for the resonator in the tailpipe.