2 1/8 to 2 1/4 primaries 36 long if you use the blue thunder or a heads that use this type of flange ?heads with the big block chevy header flange ?if not the you may want the 2 1/8 it as alot to do with what size ex you are running to. if it will be small 3 inch
then you will went to go down on the primaries size but if it more than 3 say 3 1/2 or 4 run the bigger primaries tube