Cams doubt

[Bean]

as said measure first - if the pistons out of the 3rib are in good shape and are within tolerances of the 7rib block - change the internals over with new rings and a hone and it will be as reliable as ever. This is if it can be done within spec as a fresh hone will widen the cylinder diameter.

when we are talking compression there's two things to differentiate.
static compression: the value depending on piston type, headgasket, how much block/head have been skimmed and stroke.
dynamic compression: add the type of cams to the above parameters

basically it boils down to long duration cams need higher static compression than short duration to reach the same dynamic compression because the intake valves close later and the piston is already further up the cylinder.

this is one of the more accurate calculators I found on the net, unfortunately it's in imperial measurements: http://www.projectpontiac.com/ppsite15/c...calculator
a simpler calculator for static compression (can be switched to metric): http://www.csgnetwork.com/compcalc.html

here some more into detail explanations:
http://cochise.uia.net/pkelley2/DynamicCR.html

have fun