Small block crankshaft endplay

First, I'd verify that the balancer is not moving on the crank. Re-install and tighten the balancer bolt to eliminate that possibility. If it then still has anything more than 1/16" of end play, the engine has to come apart. Hard to say what caused it. Lot's of possibilities including a ballooned torque converter (unlikely in a street car, but, I've seen it in race applications) or a burr on the cranks, faulty thrust bearing, bad machine work, etc.