Is 2 inch ok?

The fish has a 340, 650 Holley,Air gap intake, electronic ignition, not sure about the cam as we have not ever opened the motor, pretty sure it is not stock....here is the question, I have a set of TTI heasders that I will be installing soon,the exhaust is 2 inch, if I leave the 2 inch pies and exhaust on how much wil it affect the performance of the intake and headers? I am reluctant to spend the money on 2 1/2 inch if it is not that big a deal.
Thanks