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Thanks for the info, but the car picked up when I went to a custom made 2" header. Primaries are 38 and collectors are 3 1/2.



The tube size quoted may be ID not OD, if so you may be closer than you think.



Tubing is always OD
EDIT
A primary dia of 2.58 and a collector of 5"... most
big blocks dont even go that big and this is a 422"
small block that makes peak power at 6800




I gave my info to Mr P Body a while back for my 636ci and didn't get that big of numbers.;) He gave me around 2.5 primary and 4.5 collector. 410ci i'm guessing closer to 2 inch