2.5" or 3"

I have a 73 Duster w/ a 71 340 Forged crank 10 to 1 comp. Mp. 509 cam, single plane M1 , 2.02 heads 8 3/4 sure grip w/ 3:91 , hooker comp. headers.which exhaust should I go with? 2.5" or 3" ?