OK, with the piston contact with the plexiglas, sounds like you have the Silvolite or Sealed Power pistons. They are plain cast pistons so keep that in mind when you go hammering on the engine. Your 10+cc measurement seems right.
I think Wym's point may be that there is now another space with fuel-air in it that needs to be burned that may slow down the overall combustion process.
BUT the good news is that, with the 1008 gasket, you will have a decent quench gap with the closed chamber heads. That is a definite advantage. Your 9.5 SCR number is right; you indeed would be much worse off with the original late 340 pistons. A bit more would be nice with that purple cam, IMHO. DCR is in the low 7's if you stay with that purple cam; that is a part of the reason for the low compression numbers that you measured.
Whether that is a good cam or not depends on how the car will be used. IMHO, the 3 speed manual trans means better low RPM torque and a wider usable torque range is a good idea (which I am sure has been said by others). That Howard's cam is very interesting; it would put your DCR up be around 7.6-7.7 and widen the torque band some. (Note: I computed the DCR above with the duration set to 278 since the advertised duration for a solid will be with the tappet at .015" lift or thereabouts.)