Wiring 67 fastback....
If I understand your question correctly...
You want to run a 12v source from the megafuse to the starter relay post instead of running from the starter post where the battery connects.
I would say that SHOULD be fine.
As for guage I would say if it is the same as the purple #6 you would be good to go.
If you are not in a hurry, I would contact the wire manufacturer and ask the same question.
In all reality I do not see why they deviated from Chryslers design. As you pointed out why run more wires than are necessary by the headers / manifold