For all you folks out there that have fabricated your own extractors, what size primaries have you used? different engines and size (bore,exhaust and inlet ports and valves etc) will definatley all matter in the different size but how big were/was they?

I'm gona make my own as the cars off the road for another 3 months so got some time to kill and i am using a 1 3/4" sports exhaust and have had some extra swirl on the head but valves are the same...I was thinking primarys about 35 - 40 mm? And what length were the pipes going to the collector for 4 - 1 and 4 - 2 - 1 as i havent decided yet what to do..
My metal work teacher says do 4-2-1 as there better for street and below 6000rpm application as 4-1 are more for above this yes? I really need to know the length of the headerss as all pipes have to be practically exact same length for the 'scavenging' affect and no back pressure to work........

Cheers
Dave