Since I installed a Six Pack on my 69 GTX, the power brake booster doesn't seem to be working properly. I now have to press a lot harder on the pedal to stop.

1- Could the installation of the Six Pack affect the functioning of the power brake booster?

2- If not, could the power brake booster be defective?

could be either.
you need to see how much vacuum you are making to see if it is the booster or the new intake.

it is manifold vacuum, so if you didn't change the cam, I would be suprised, but don't know enough about 6 packs to say.

I agree, it looks like a vacuum problem. I guess you need to know what the minmum is. My system requires 14 to 16 inches. I think anything less than 11 inches and you will have issues. See if you can get a vacuum measurement. I just read in my manual that the booster itself is not repairable.

I did change the cam: Xtreme Energy™, XE262H. If this is the problem, is there a solution?

they make vacuum canisters. you hook those up to "store" extra vacuum for your brakes.

electric vacuum pumps are another choice.

hydro boost uses your power steering to supply assist.

or manual brakes.