340 questions

Hi everyone,
I am new to the forum and have a few questions about my 340. I just purchased a 69 Swinger with a stock rebuilt 340 in it. The problem is the guy I bought the car from was the 2nd owner after the restoration and doesn't know what pistons were used for the rebuild. Is there anyway to check and see what compression the motor is with it in the car? I know there are some formulas that involve the combustion chamber volume etc but like I said the motor is in the car. I know from looking at the harmonic balancer that the crank is cast. The reason I am concerned is the guy sent a new .484/284 cam w/adj rockers with car. I would like to use this cam but I am worried that they used low compression pistons for the rebuild and I would like to know before we go through all the work. I want to put the car into the 12's, I have the above mentioned cam, a edelbrock air gap intake/holley 750 to go onto the car. I would also like add TTI headers and to ditch the 3.23's and go with 3.91's and a stall. The car has J heads and I am not sure if they are the 2.02's or not. Sorry this is so long just a new guy with a lot of questions :book: Thanks for any help and suggestions!! :salut: