Let me add a "me too" to the suggestion about patching the leak with epoxy. The pressure in the cooling system is so low on one of the old 216's that you won't need a pressure-tight seal like on a newer engine, and welding the block will require a complete teardown of the engine, prteheating, peening, post-heating, and a lot of other monkey motion, and it still might crack beside the weld. The "sewing" procedure with the threaded plugs works best with specially designed threaded, tapered cast iron plugs, and the holes need to overlap to work properly. Drill & tap one hole at a time, thread the plug into place, cut it off, and then drill the next hole overlapping a little.
The epoxy fix can be done with the engine in place, by simply sandblasting the cracked area with a portable sandblast gun, blowing the dust away, and then spreading a liberal coat of epoxy over the crack. Be sure to drain the coolant first. Before it hardens, press a piece of 40 grit sandpaper onto the patch to simulate the rough surface of the cast iron. The repair will almost disappear, especially after it's painted.
Jerry
