Wiring 67 fastback....

"0" wire sizes ---------- Wire gauges as I'm sure you know get larger from such as 18, 24 (larger) 10 (larger), 6 (even larger) down to 1 and then 0 and then the more "o" (ought) the larger the wire, so 0 is larger than 1 which is larger than 2, and "00" or "double ought" is larger than 1 and larger than "0"

No.6 to the relay won't hurt. You can use that stud for a junction to add things "hot" with battery so having a large feed to it will not hurt.