70-100lb bag should be ok, especially if you have a floor mount hook and a bungee to keep the bag from swinging. Ill post pics of how I hung the Heavy bag I have, in my garage as soon as I get a chance. I dont have exposed studs in my garage, so I had to use a stud finder, and install a unique but very effective setup, especially considering the ceiling height is low. All of the parts I was able to pick up at a Sears Hardware store. Ill list it out and post pics later tonight for you.