The horsepower of your engine needs to be known or assumed to configure a appropriate headder design for your car. If you would like some basic dimensions I will indulge you. The bb3 extra will like 2.25" primarys off the head. You might step the headder to enhance the top end horsepower with a step if no good info is known on your engine, 1/2 way between the head and the collector is a good starting point. The primarys may like to be 26" to 28" long from back of exh valve to collector. a little longer than that will only help the bottom end torque. As for the collector I like to use Merge collectors. Using the step pipe to 2.375" at the collector should have a 4.5" outlet 12" long if you are making big Horsepower with a choke maybe of 4". I like to use SPD for the collector. Chris Hill is the owner and is invaluable to me when I need help designing headders that work. I would not go buy parts based on this write up alone. The engine power #s are the ultimate deciding factors in the collector sizing. The headder flange and how it fits the exhaust port is the money shot and deserves some attention to get the fit right the first time. I take plane paper and rub a pencile no its side to transfer the exact port relationship to the fasteners to make the flange fit correctly. Believe it or not if you choose but every cylinder head reguardless of whom makes it often is as unique as a snow flake in the aftermarket head business. There are so many different port designs and porting jobs that change often you don't know what will fit untill you know. I realise that I did not answer your question the way you may have hoped, but no one can without more info on the engine actual power and RPM you plan to buzz. I hope this gives you something to work with for starters.