You won’t get there with 6X-8 heads.
Even the 6X-4 heads need a pretty fair amount of milling to get up to the high 8:1 range.
Finding some ‘68 #17, 69 #47 or ‘70 #11 350 heads would be a good way to go, you can get them from cheap to free. Other than the press-in rocker studs they’re pretty much ideal for a street 400. The smaller 1.96”/1.66” valves are fine for a hot little street 400, if you want to upgrade to 2.11”/1.77” valves that’s easy to do too but really not necessary.
The chambers are probably in the low to mid 80cc range before milling, it should be pretty easy to get the compression somewhere between 9 and 9.5:1.
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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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