This is the way I see it.
The plate should be the same size as the exhaust port on the OEM manifold. This will create the same exact flow as the 0331 head. I will be using a 7120 head since thats what I have laying around (thank god Im a parts horder). The plate costs $110 shipped, and the head gasket, manifold gaskets, head bolts, VC gasket, etc costs $108 shipped via Rock Auto which is much cheaper than the big box stores around me.
If there was a feasible way to get around the precats (2-4 O2 sensors, not sure how many I have yet) Or I could use a 02 sim that takes the place of a sensor. The downpipe would be a problem unless I retro in all pre-98 exhaust which actully might work.
I wish I had more manifolds to play with, all I have is my current 2001 (split header) and a 1995 style long tube.
When I get time I can connect via laptop to the ECU to see if I can turn off the 02 cel light where the engine wouldnt go into limp mode. I would just need to see if anyone dissasembled the ECU code yet, if not I have a friend in AUS that is willing to help out.
The 02 sensors Im more concerned with are the front ones, I know I have 2 of them. These are the ones that dictate the STFT, LTFT trims during a WOT pull, or during regular cruising.
Now that Im rambling here, I wonder if I can just weld in a 2 02 bungs in the 95 manifold and just completely get rid of the precats??? The earliest I can do it this way since the XJ is my DD till spring/summer, but I can figure out where I need to stick the bungs before hand so its ready to go if I do decide to take the time to do it.
Here is the 02 sim I was talking about earlier:
http://www.ubertechnics.com/o2_simulator_2.5.aspx
Now Im almost positive that if I use the sim Ill throw a code for the rear 02 sensors saying some BS about being too rich/lean. The trick for that is to drill out a 1/2" spark plug anti-fouler so you can bring the rear 02s out of the exhaust stream...trust me this worked on my Evo for 50k miles.