I have a 71 roadrunner with manuL drum brakes. Can i buy a brake booster at a parts house and it be a direct bolt on deal? Or do I need a different brake pedal pushrod? I work at oreilllys and can get a setup in a day. Just want to be sure that this is as easy as it seems.

The manual linkage/rod is different than the one for power drum brakes. You will need to find a car that has a power drum brake booster and take the linkage off it.

Sorry forgot to mention im putting on a-body disc brakes.

I would suggest getting an adjustable proportioning valve

What GLR said, you need the whole power brake assy off of a donor car. Likely the booster is still good so just replace the M/C (unless the whole assy is fresh from a functioning vehicle) then you're all set at the firewall/under dash part of it (not sure if the brake pedal is the same or not).