I can't remember what the stock L20B pipe diameter is, may be 2" but there's no mention of cams, head work and induction. That said I doubt it will flow as much as a 3.5 liter engine. The cross section of a 1 7/8" pipe is 2.75 square inches. If you go to a 2.5" pipe the cross sectional area jumps to 4.9 square inches. Now that's an increase in area of 77% so if a 1/78 pipe works well enough for a 2 liter engine a 2.5" pipe will support a 3.55 liter engine.
Mike, as working at an exhaust shop for 4 years I can agree with you on this. We are one of few shops who only do SS and bend manderal in house. We gurantee 3" manderal tubing for up to 600hp so it would depend on the numbers we are talking, but yes 2.5" should be enough for a 3.5L engine that's NA. Many engines up to 5L even use 2.5in tubing, really the displacement doesn't mean much as far as tubing size is concerned. Just HP and Torque. Or if turbo charged.