Funny you ask. My old 383 combo was almost EXACTLY what you have going.
It was a vortec headed, 325hp, 430lbs/ft 383. I had 1 5/8" into 3" collector full length headers with dual 2.5" exuast on it. Pretty dang close huh?
Yes I think 3" would be better for better overall tract times. But if this is a daily driver/towing motor then I think Id stay with 2.5"
The only problem I have with this exaust combo is the size is going in the wrong direction from collector to pipe. You want the oriffice the gas is coming out of to go into a bigger area. Same idea as step tube headers. They start small [like 1.5"] and go one step bigger [to 1 5/8"] so I would rather use 2.5" collectors into 3" tubes.
But with the above combo, and one dead cylender [dont ask, the cam lobe was flat for years before I knew and before I bought it] I could yank the earth off its axis. It was an absolute torque monster at low RPM's. I could start from a dead stop at 1000rpm no prob. I think you'll be very pleased with it.