I would keep the original 093 heads on your '66 GTO Tri-Power engine. Run a set of dished pistons to lower the compression. A 15cc dish will get the compression down close to 9:1, enabling you to run 91 octane premium unleaded fuel.
A few gallons of leaded racing fuel from time to time will be enough to coat the valves and seats and keep them happy.
If you decide to use the 6X-4 heads you'll end up with between 8 and 8.5:1 compression depending on bore size and how much you mill the heads. You'll have to mill them pretty heavily (about .060) to get close to 8.5:1.
In my opinion it would be worth the extra $$ to have a set of pistons custom-made (Ross, SRP) to your final bore size with the 15 cc dish and valve reliefs properly located.
When you open the hood the engine will look 'right' with the 093 heads. Your cam choice won't be limited by a set of pistons with valve reliefs that don't match your heads.
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1964 Tempest Coupe LS3/4L70E/3.42
1964 Le Mans Convertible 421 HO/TH350/3.08
2002 WS6 Convertible LS1/4L60E/3.23
1966 VW Type 2 Single Cab Pickup 140hp 2276
1956 VW Type 2 Panel Van 36hp 1200
1957 VW Type 1 Beetle 180hp 2276
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