Has an effect on the spine of an arrow? I think so. If you have 2 seventy pound bows but the ATA is different doesn't the longer one increase poundage more gradually.

I haven't shot any of the shorter bows so I have no experience to go on, but my 38.5 ata bow with a 340 arrow for the most part always seems like it's to stiff.

If there is some truth to it that would help reduce the guessing about which arrow/spine is best.