I need at least 1 7/8 primary due to my port on the heads, that's as small as I could go. Personally I see no need in a 3 inch crossover. I'd rather keep velocity up and save the space with the smaller 2 1/2 crossover. I have seen people use 3" as a crossover but you have to cut that size pipe way down to fit it to a T6 flange. 2 of the 2 1/2 pipes fit nicely to the T5 flange so I can't see the bigger pipe as a gain when the restriction is the turbine inlet.
My head flanges are 1/2 thick and work great. You don't have to use a gasket and they seal without warping.