[cycleguy55]

A pair of 2.25" tubes has more cross-sectional area than a single 3" tube (3.98 in² x 2 = 7.95 in² vs 7.07 in²).

How about building headers with paired cylinders (similar to Tri-Y headers) and run a pair of 2.25" tubes back to mufflers with dual in / dual out (quad tips) - something like the Magnaflow 11378? Alternatively, you could run dual in / single out (e.g. Magnaflow 12158, 12258, 12278) for a more period-correct appearance.