Here's an easy way to check the slack in the chain: take a breaker bar and 15/16" socket to the center bolt on the crank. Remove the distributor cap. Using the breaker bar, rotate the crank back and forth and take note as to how far the crank moves before you see the distributor rotor move. I don't have a spec as how far is too much, but I'd say if you can move the crank more than 10-15* I'd replace the timing set pronto. 90* movement of the crank is one full cylinder in the firing order.