No worries - so long as the thread gets going . . . Sounds like a great project!
I am a street guy (stuck in the early 50s') so I can't offer much in the way of experience here.
But I find the work of David Vizard instructive
Exhaust Science Demystified. His work suggests that the primary tubes on the siamese ports most definitely need to be larger because they will carry twice the volume.
The article includes a chart showing header primary tube sizes based on CFM flow rates. The displacement of a 261 puts it on the lower left side of the chart in the neighborhood of ~1.25" primaries.
His experience suggests that getting the collector sized right (diameter & length) is just as important as the primary pipes - and that bigger is not better. His experience suggests that the collector not be left open - and that a track-tuned length of secondary piping is needed to finish the job.
Send me a PM. We should be able to get a photo in the Rides section.
regards,
stock49
