The engine was not in a Pantera. It was in a Mustang. D1AE Ford Service Boss heads as delivered, uncut.
I believe that the actual CR was 10.7 withe the pop ups and closed chambers and 10.3 with the open chamber heads.
Part of the issue was the domed pistons, These were TRW's +.030 with fire slots.
Those pistons are actually very well designed but they were intended to use the old 106 leaded fuel. They don't particularly like the unleaded fuel including the Sunoco unleaded racing gas.
It was no better then 93 pump premium.
That is all just ancient history.
They don't work as well with the open chamber heads either. The fire slot is for flame travel and the chamber shape change effects that. Don't bother with them.
The current flat tops with closed chamber heads are really close to a problem with Premium pump gas now. That set up would be better with closed chamber aluminum heads where you limit the total advance to about 32° instead of 36°. The dished are actually more practical with the close chambers for a street car with iron heads.
There are more ways to make power then compression ratio. In fact many aftermarket camshafts will bleed off compression through the exhaust valve intentionally.
If you match your components correctly, you can easily make 650 to 700 hp in a 5.7 Cleveland with a 9.5:1 cr.