Exhaust sizing advice

[Flash68]

I picked up these headers off ebay a couple months ago for my blown 406 Camaro (650-700 rwhp). We are getting ready to have the exhaust done soon, so I am wondering what is the best route to go. The headers are stepped 1-3/4" to 1-7/8" primaries with stainless non merge 3.5" slip on collectors (per the seller). After signicant research on this forum and a couple others, I am thinking this might be a good route to take but am no expert by any means.

How about 3.5" pipe from collectors to X-pipe, then 3" out of the X-pipe to 3" mufflers (that's a whole different topic) and then exiting over the axle behind the rear wheels?

Or would doing a 3" system from front to back be the best? I figured the 3.5" to the X would be a smoother transition from collector on back?

Thanks in advance.