You can use a short length of all-thread & a couple of nuts with the same thread as your balancer bolt (I think BBC is 1/2" fine-thread). Double-nut the all-thread at one end & thread the other end into the crank as deep as you can get it. Remove the nuts, install the fat washer, then a regular washer or two with grease in between, then a nut (grease the threads). Tightening the nut should press the balancer the rest of the way on. You'll need to find a way to keep the engine from turning while you tighten it.
I got a 3' length of fine-threaded all-thread at a local bolt store (found in the yellow pages under bolts), & cut it to a convenient length. Most hardware stores only had it in standard-thread.