Lose the 2.5 inch pipes, they are a really big restrictor on BB motors Been there done that
Run 3.0 or bigger from the collectors to the end of your new system, I used a set of the early Magnaflow 3.0 side inlet with the 3.0 inch center outlet by 14 inch wide by 7 inch high by 16 inch ( I think, maybe they where 14 inch long) mounted at the rear bumpers on my old pump gas 400 stroker motor, that car picked up .02 ET and .2MPH in the 1/8 mile removing the complete exhaust system, 7.01 ET 95.5 MPH versus 6.99 ET with 95.7 MPH without the exhaust on the car. I think the difference was in the weight, not the exhaust restriction
With those mufflers mounted behind the rear end and exiting at the rear bumper it was fairly quiet while cruising and louder at WOT but not like trunning he car with headers only