Engine Help

Hello, I have a 71 240z. Problem was car quit on me 2 years ago. Been tinkering with it since. Replaced all fuel lines, fuel pump, plugs, wires, coil, resistor, all wiring, cleaned carbs. Finally got it running. It purred for about 1/2 hour. Shut it off then next morning did a oil change, radiator flush. Now my problem, after changed fluids i tried to start and imediatly started knocking hard on the passenger side, right around the bottom of the cylinder head area. With the knocking it wont turn over hard enough to start, sounds like a hammer on the side of the block. I am a rookie when it comes to engines. cant figure it out from here. any thought as to what it might be will be helpful .Thanks