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