FYI, no engine is going to see an improvement in power by increasing the back pressure, that's an old wives tail. The full exhaust improved the overal tuning of the system to allow the exhaust to more efficiently get out of the cylinders compared to the open headers.
To the OP regarding tube size. I had a dual 2 1/4" system with an H pipe cross over on the cuda when it had the \6. When I swapped in the 360 (~370H) I just adapted those pipes to the 360's headers and ran it that way for about a year. I kept saying I need to upgrade the exhaust so my daughter installed a 2.5" system (she was running her grand father muffler shop at the time) with an x pipe and 50 series flowmaster mufflers. By the butt dyno there was no difference in power. I wasn't happy with the way the tail pipes were bent (they hit the shocks on big dips in the road) so this past winter I installed a 2.5" system that came with mandrel bent pipes, an x pipe and turbo mufflers. Still no change by the butt dyno.
IMHO, you will feel little or nothing by increasing the pipe size or changing the mufflers.