I was wondering what the correct compression should be for a 350 rocket out of my 71. And if possible could anyone tell me how to change it if i need to?

 

8.5:1, if you are lucky. More likely around 7.9 if you do the math.
That engine has some seriously dished pistons, you will need flat top pistons, or another shortblock with early pistons in it to raise the compression to a decent level. I would shoot for around 9-9.5:1.
The cylinder heads are keepers (#7's). Good chamber volume, Decent ports, just need some bigger intake valves correctly installed in them.
What is your plans, what are you going to do with the car?
Is it a low rider, a putzer, a cruiser, or do you plan to drag race it?
JIm