Crankshaft Pulley Bolt missing

I'd use a set of bolts that are all the same. Not a big deal, but, I like keeping the rotating balance even. As for your problem, it may be that there is a mismatch on the balancer and pulley bolt pattern. '71 and older systems used one offset bolt where the pulley would only mount on the balancer one way. In '72 they started using an even bolt pattern where you could bolt the pulley on 6 different ways. The solution if you have a mismatch? Just use a rat tail file and slot the one pulley hole that doesn't allow the bolt to install.