Thanks for the kind words
The best way to do it, if we absolutely had to use your NA muffler, would be a gradual reduction in diameter. That's going to be very difficult to accomplish within the catalytic converter without a pretty spendy stainless reducer. You'd want to use stainless rather than the super-soft low-grade mild steel reducers most parts stores carry. The better option would be to reduce the diameter at the cat outlet rather than the inlet; the exhaust gases have to flare out from the inlet pipe to the catalytic element, whereas at the outlet, they are converging toward the outlet spout anyway. The outlet is thus the more logical place to reduce pipe diameter.
If I were in your shoes, I would use a 2.25" cat and just live with the mismatch where the cat meets the muffler. It is not ideal, but it does allow you to upgrade to a better muffler down the road (HKS SC, or Toyota SC, or one of my own design if that ship ever sails). If we build the cat 2.25" all the way through now, it'll be a better match for the bigger muffler when you get to that point.