You could try a different balancer installer - the one I use has a "plate" or flange that would have covered your recessed hole completely so that both nuts remained outside where you could reach them. It is an .

On the other hand, using the crank bolt also works like Lariat just mentioned. You just have to keep the pistons from moving. If the oil pan hasn't been sealed you can drop it and stick a big screwdriver between one of the crank counterbalances and the block. Or if you did already seal it, thread some rope down the spark plug hole of one of your pistons until you fill the cylinder. As you tighten the bolt the engine will want to turn but the rope will prevent the cylinder from going up.