If your piston is too close to the head, you'll get piston slap. Smaller heads can cause detonation because they leave trapped pockets of fuel.
If you plan on using a 50cc head on a 70cc kit, either have a buddy with a lathe turn your head so that you have the proper squish or get comfortable with a set of files and do your best to make it smooth.
If your head has a centered spark plug, you can chuck the spark plug in a drill, thread the head onto the plug, and use some sandpaper to turn down the squish. If you have an old 70cc piston, you can always glue some emery cloth to the top of the piston, and that will give you the correct shape.
You don't want any sharp edges in the combustion chamber.