How to measure the length of the header tubes?

[Enzo Bertaggia]

I have read that the original GT40s had a header tube length of 36". Is this the distance from the flange at the head to the start of the collector or ...?

I have heard conflicting opinions on whether the length is the tube length (which would include the tube that is slipped inside the collector) or whether the end point is where the merge point is inside the collector.

Thanks,
Enzo