The threads are 5/8-20. It is best to go with a steel braided line which would be a No. 8 AN. The part number for a No. 6, 5/16 is RUS-640273 in the Summit catalog. I have never seen larger adapters for these carbs but if they exist, the tech guy at Summit can help you quickly. I have always found
No. 6 to be very efficient with these carbs using a 3/8 line into the inlet side of the fuel pump. AFBs and Edelbrocks do not like fuel pressure over 5-lbs. It tends to over power the needle and seat and fuel will weep out the boosters at idle causing it to run like poop. Hope this helps.
Pat