Slow crank means poor ground. Your engine, if properly measured and assembled is not 'to tight'. Since its a 216 I'll *assume* ots still at 6v? 6V anything needs fat electrical cables (to the starter (like 000-0000 size) and clean, clean clean grounds.
If your at 6V put a 12V battery on it (make sure all electric gauges are out/unhooked) and use that.
IF you are at 12V now, take the plugs out and spin it by hand... it should spin with no compression.
