On your deal are you running a SB or BB?
If BB I would go to a three inch exhaust and not waste my money on enlarging the system a 1/8 inch
I have a BB Dart that I use to bracket race, I saw a ET decrease of 12.23 to 11.39 in the 1/4 mile running the car with a 2 1/2 inch system with a set of the old Corvair Turbo mufflers with side pipes exiting in front of the rear tires to removing it
It went from 109.5 MPH to 117.7 MPH with at E.T. change also with no other changes
My last pump gas hot rod had a full length to the rear bumper three inch exhaust on it, it change from running 7.02 ET at 95.4 MPH in the 1/8 mile to 6.99 at 95.6 MPH by removing that system
The exhaust system size can matter