For your deal I would buy the two inch headers with 3 or 3.5 inch collectors and put a 3 inch exhaust system on the car to start with now instead of later
You've already made the motor bigger for more power, correct
Do the same on the exhaust now instead of later, it always costs more later
My old 440 powered Dart picked up .8 ET and 8.+ MPH in the 1/4 mile by removing the two and half inch exhaust system from the car
It had the old Thrust 2.5 inch Turbo muffs hooked up to the Hooker 1 7/8 Hooker fender well headers with 3.5 inch collectors back about 18 inches with 2.5 inch pipes to the Turbo mufffs and then around 18 inches more of 2.5 inch pipes out to the side exhaust Very restrictive