The Dr. Gas synchronizing crossover pipe system installed on my car is made of 3" mandrel bent aluminized tubing leading from the Hooker Super Competition headers to the "X" and 2-1/2" from the "X" back.  The system uses a ball and cone style joint just in front of the Super Turbo mufflers so the X-pipe system can be easily removed without cutting.

Because the system was made with mandrel bent pipes, crimping of the pipe was avoided.   It is the crimp, or narrowing of the cross section of the exhaust  tubing, so common with muffler shop pipe benders, that  restricts exhaust flow.   Once the exhaust pipe is crimped,  effectively reducing its diameter, flow from the crimped area back is reduced no matter how large the system is after the reduction.   Not shown are the Torque Tech 2-1/2" mandrel-bent tailpipes.