Doing it right -need some advice

You don't need to know the compression ratio at this point. Even if you knew it, the Scr by itself would be relatively meaningless.
What you really need to know is the cranking cylinder pressure; it will tell you everything you need to know; save yourself the cost of the gaskets.
Here's my take on pressure for iron heads;
under 125 is just extremely poor.
125 to 135 will make a ho-humm streeter, but your starter gear of 3.09 x 4.10= 12.67 will mitigate that.
A modest minimum target for your gears would be 140 to 150psi
150 to 160psi is good to very good;
over 165psi is probably bad.
Your Second gear ratio is 1.67 x 4.10 =6.84; which is the same as mine (3.55 x 1.92=6.82), so I know it will be fun. The more pressure you have, the more fun it will be, until it gets into detonation. Your overdrive ratio makes 65 ~2440, which should be about right for a 340 with a 4-speed cam..