By and large, most of the USRRC cars, the 5 FIA cars, and the earlier Sebring comp cars all had 3" main pipes, with 3.5" fake muffler cases.
I say "by and large" because there was so much variation in the sidepipe design and layout, from car to car.
Looking at vintage photos of the 289 comp cars, I see some that look like they just had 3 inch straight pipes, and then there's CSX2557, which had an early variant of the 2 into into one collectors, with dual sidepipes, that were eventually used on the Daytona Coupes
When I was re-doing my car's sidepipes, I found this diagram which helped me a lot. I can't remember where I downloaded it from, so I don't know who to give proper credit to, but it is a great sketch:
Attachment 29885
I think this drawing was taken from the pipes on CSX2345.
I ended up basing my pipes on these two old photos. I liked how the pipes tucked up under the rockers (no snake bites for the kids
)
Attachment 29886
Attachment 29887
Jeff definitely knows more about the original header design than I do - Listen to him
