difference between headers and mid pipes.

so i plan on buying an H-Pipe for my mustang, and headers eventually.
but i saw that some Pipes say GT Only.

does the tubing on V6 and V8 headers have diff diameters or something?
or will i still have no problem installing a GT deigned H-pipe on a V6 Header?
can someone explain in more depth?

i plan on getting probably long tube headers, o/r H-Pipe, and i havent decided on the exhaust yet. ive heard American Thunder sounds better than Borla.

anyways, please help with my question. thanks.

and with an X or H pipe installed, i can install a GT Exhaust no problem right?