If you keep South blocked, you can just camp by your home, and collect tanks occasionally from the three places they drive to before running out of fuel. Tanks to the south can shoot you before you see them: to north, west and east, you can see them first (at least towards the top of your screen).
Unfortunately, this strategy makes the game tedious and less-fun. My best so far is 96, and I doubt I have the patience to try again to reach 100... I only died because I'd got bored and was doing silly stuff without recharging hearts.
At most 28 health hearts fit on the screen, but I have found no upper limit of health: if you have more than 28 health, they just go off the edge of the screen, and no hearts will disappear when you are hit until you are down to 28 health again.