An engine will not run at all with the timing 180 out. The normal firing order is 1-5-3-6-2-4. Yours sounds right- - - -no stumble or miss, just a very loud mechanical noise. To check the firing order, pull the #1 spark plug, put your thumb over the plug hole, and have someone tap the starter a little at a time until you feel a puff of compression. Pull the distributor cap and see which cap terminal the rotor points toward. That's #1. Follow the wires in a clockwise direction from the top, and make sure they fit the firing order above. It makes no difference what cap position is the #1 wire, as long as the wire aligns with the rotor with the #1 cylinder on compression.
Edit: What's the chance something has fallen down the carburetor and gotten imbedded in the top of a piston? Did anything fall into a spark plug hole while you were changing plugs, maybe? That sounds like something is hitting the cylinder head every time one of the pistons comes to the top, or there's a VERY bad rod knock. Try taking all the plugs out and turning the engine by hand to see if there's a spot where things get hard to turn. Did the noise begin immediately after the plug change, or was it there before you started tinkering with the engine?
BTW, that bent fan is a disaster waiting to happen. It would be a very good idea to change it.
Jerry
