There are specific ways to install freeze plugs without warping or otherwise distorting them so that they won't leak. This is especially critical with softer brass plugs. Never hit the plugs directly with a hammer. Many professional engine builders will have punches sized specifically for the plugs, but you can do the same thing with a large socket. For Chevys a 11/8 socket is just about right. Try to drive the plug in square with the bore and just until the lip of the cup is flush with the block.