If you are changing the resonators, the pipe size is not a major issue. Bigger is better in that area, at least up to a point. 2 1/4 to whatever adaptors can be found at any local parts store. You can weld them or clamp them...whatever suits you. The pipe can also be stretched to fit over, or into at least 1/8 inch larger tubing. i just did a custom dual exhaust for my OCC in 2 1/4 inch pipe. Mine exits with turn downs just in front of the rear axle. The difference in flow is substantial about .8 square inches of increased size, or 25% more flow. for the 2 1/4 inch pipe over the 2 inch pipe.