I can promise you that most if not all attach screw holes in RV-6 tanks are different.
Don't know if the later "pre-punched" skin -6s punched the tank attach holes or not.
I DO know that the older ones HAD to be "adjusted" from the plans considerably. The original RV-6 plans were "converted" from RV-4 plans and the tank attach holes did NOT correlate.
If you change tanks, you will have to drill the new tanks to match the wing.