How do I calculate compression or guess

If this is to satisfy Big Brother - I'd grab a couple of running 318s for now. Anything really that will bolt in unless you have the cash to complete the two builds simultaneously. In regard to the pistons - Any open chamber head will work fine with them. Even one milled a bit. I'd expect a set milled .040 to be in the 70-71cc range. I have never, ever seen a set of factory original heads that was at the spec of 72ccs when measured. All were larger, some by 4-5ccs. Regardless, the pistons will be below deck in any block. Below deck by a bit (IMO) in a stock, non-blueprinted block. So if it's a concern of too much static compression - you really have no concern. The other deal is the gaskets. You have choices from .028 or so through .050+ depending on the brand. So again - you have no worries. Order them up and get the engines built if you have the means.