I used (3) 3" stainless pieces to make the crossover pipe. Since all the ports just dumped into the center section, I figured I could tie them all together with exhaust tube, so long as the exhaust pressure could mix efficiently.
IMPORTANT: take the two seperated exhaust cans and mount them back on the car first. Use all the mounting bolts as if it was final install. Make sure the exhaust tips are aligned, and everything looks good. Now measure the width between the two cans - this is the only way to be sure you'll get a final product that aligns those tips.
Mine measured out to 8" even - and I noticed that there was some spring tension that caused the back half of the cans to measure slightly wider than 8", and I could see that in the alignment of the exhaust tips. By making the x-pipe 8", it would pull everything back into straight alignment. Please don't just use my measurement - who knows how your car may differ!
In order to achieve a free-flowing x-pipe, I cut baffles into the two outside tubes (shown is first side). I sectioned the center tube lengthwise to help size the baffles, but eventually I just cut the other side of this center tube as well, producing two rounded center pieces to tie it all together.
Before cutting into the tubing, and in order to make sure the x-pipe ended up wide enough to cover the two 2.5" ports, I stood the two outside tubes on the garage floor, and then stacked the center tube on top. This let me shift things around until I had the width measurement I needed - about 7.5".
Here's the x-pipe before welding all seams.
Now, take the x-pipe back to the car. Loosen mounting bolts enough to be able to slip the x-pipe up in-between the two cans, then go back through the process of tightening and aligning the exhaust tips - this is your last chance to get the alignment correct - once its welded in, you get to look at your handiwork every time you walk out to the car, so take your time, get help, get it right.
Use Sharpie to make alignment marks on the side of the can & x-pipe. I chose not to tack weld the x-pipe in, because I wanted to make sure the fit was flush to the can. Once you have your alignment marks, you can take the cans and the x-pipe down to the bench & weld 'em up.
The final step was to measure and cut a piece of the 1.5" tubing to cross connect one of the smaller engine-side ports (these are the ports that don't have muffle material visible, and are closest to the flange). I chose the port closest to the x-pipe, because it looked like I'd be able to weld it all up without leaks. I had to grind off the lip of the port in order to fit the tubing between the cans. I chose to just patch over the remaining 1.5" port, mostly because I couldn't see how to do a decent job welding the seams. I can't speak to the outcome, but I imagine I could have just patched both these holes and let the extra pressure go to the x-connect section, but I didn't want to create too much back pressure.
Another look at the patched port. The 1.5" tube is just out of view.
Here's the whole unit, before doing the smaller cross connect. Note: I *did* run it this way for a few days, but it was too loud in my opinion - good for a bit of fun, but not what I was looking for. Plus, the low-mid torque felt very mushy with the engine-side ports still open. The two "top" ports (you can see the fiberglass material) will be left uncovered - the material has stayed put so far
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That's pretty much it - mine bolted right up with no fuss and the tips look aligned. Firing it up was much quieter than running around with the engine-side ports opened, and after driving it a bit that torque was back big time. The exhaust isn't crazy loud, but is notably louder than the stock Nismo. Its not raspy, there's no drone and I still hear that off-throttle between gears stumble-kinda-misfire sound that I've really come to like.
Hopefully if you try this mod you'll like the results as much as I do. At this point, I don't plan to upgrade the exhaust - though I may get some high-flow cats for even more flow.