360 piston id and compression test results

I have a good running 1971 360 in my dart that i dont know anything about and recently did a compression test. With the engine warm all the cylinders were roughly 140-150 psi. I stuck a bore scope in the spark plug opening to see what kind of pistons i have, i saw a flat top piston with no valve reliefs and the number 30 etched in the center. Is this a .30 over cast piston by chance? Is my compression ratio roughly 8.1 with the 140-150 psi hot test? My vacuum gauge recently died but i remember the car idling at roughly 16-18inches to give an idea of cam. Im thinking of a cam swap in the future with some better flowing heads (magnum) for this la 360 with the intent to build a stout magnum short block down the road and transfer the heads over. I know ill have to change springs when going from flat to roller cam.