Difference in Early and Late dual exhaust?

67 is a weird animal; actually different from 68-up if you want it to fit right due to the different fuel tank hanger arrangement. Don't know how a 64 compares to that. I suspect your best bet would be to go to your local exhaust guy (not Midas — the hole in the wall place the local motorheads use) and have him fab up a dual system. Pretty sure it would be cheaper than $800.