There are lots of calculators for compression ratio, but they all require the exact
measurements of the engine to give you an accurate number.
You can guess on a few, but it is just a guess.
The piston is the biggest factor.
Most 67 390's where in the 10.5 to 1 range.
For the most info you would have to give all the specs you can for your engine.
Here's a whole pile of configulators.
http://www.wallaceracing.com/Calculators.htm
