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The pictures show everything quite close and I an sure of the fit up mating to a stock catback which is the first issue. I don't know if theres enough length. I don't see the flanges at the stock location so what you show may not even work. The cats length is as big of an issue as width, which isnt truly solved with your cats.
Also everyone has their opinion but I have yet to see any evidence of serious improvement over the y-pipe.
Yes, I wish I had better pics, and I'll try to get some better ones once the '65 Mustang comes down off the lift next week, but I don't feel I'm blowing my own horn too loudly when I say the packaging is light-years better than any production or aftermarket catted Y-pipe I've seen.
This is a side view that perhaps shows the front-to-rear layout of the cats a bit better:
http://www.milleredp.com/gallery/v/jem/motion/SHO/exhaust/DSCN6037.JPG.html
Note that the fat part of the rear cat is entirely forward of the steering crossmember, the crossmember passes over the exit cone of the cat.
In this picture I'm holding the flex approximately at the point the olive joint occupies in a typical Y-pipe configuration:
http://www.milleredp.com/gallery/v/jem/motion/SHO/exhaust/DSCN6044.JPG.html
Note that I've got just under a foot of space in front of it with which to effect a decent merge.
The key to all this is that not just that the cats are shorter (they are) but that with the exception of the V-band ****** at the front header everything is welded, and everything was pretty much cut down to minimum length; there's no 2in long slip-joints, generally no room for them, and you could never do this as a clamp-together design. The center section drops off for axle, etc. work with the two V-band clamps and the two-bolt ****** at the rear header but if a cat ever needs replacement I'd have to cut and weld. That suits me well enough because "clamped slip-fit joint" is usually a synonym for "leak" and if you ever get the clamps tight enough not to leak, rattle, or work apart you need an angle grinder and a sledgehammer to get it apart anyway.
I won't ever have the opportunity to do a direct A/B dyno comparison of this configuration vs a stock olive joint (though this setup is intended to let me put a flex and V-band ****** on the rear of my old SHO Bros "equal length" Y-pipe and bolt it up with stock manifolds should I get paranoid around smog-time) but a Y-pipe intended to bolt up to a stock catback has to have a spigot that will fit inside the OE 2.25in catback, which on the ones I've seen means an effective Y-pipe exit diameter around 1 7/8in.
Now, I'm not in the aftermarket parts business, and I'm not criticizing those who take a different direction in order to maximize the 'bolt-in' appeal of their product (particularly given the limited market for something like a SHO Y-pipe) but I think the most effective way to approach this (given, too, how big a maintenace headache those olive joints can be with the slightest pipe misalignment) would be a Y-pipe with a 2.5in exit, a 2.5in flex, and a 2.5-2.25in reducer to clamp or weld on to the front of the catback once the olive joint ******'s been cut off.
