According to Clifford Performance the 4 / 6 cylinder specialists 2" exhaust is mint for 2.8 - 3.4L 'stage 2' tuning. Any larger and your wasting precious torque unless your running some twin turbo setup that requires dual 3" exhaust.
I have WCF shorty headers running into 2" mandrel tubing. The firewall side comes up and over the transmission and dumps out the drivers side exit under the facia. The rear header exits and does a 135 degree turn and dumps out the pass side. total cost to do this was 500 CDN, not including the mufflers. both pipes end in supertrapps. if you have any questions, let me know.
1984 Indy Fiero 4spd 3.4L
-Est 220hp @ 5000rpm / 230 tq @4500rpm, centerforce II clutch, 390cfm 4bbl holley, crane h272 cam, Edelbrock Torker II intake, 9 to 1 comp, minor head porting,headers, true 2" mandrel dual Supertrapp exhaust, factory 4.10 4 spd, 17x7 white Konig Ziege FR 205/40R17 RR 235/45R17. All interior leather changed to white, custom built adj aluminum rear wing, aftermarket gauges, corvette MBC upgrade, Held coilover kit, 2 inch drop front and back.