Back in the 70's I had this very problem but I wasn't smart enough at the time to understand it .
Here's the deal if you tighten the axle nut without a spacer between the two inner races it forces the inner races towards each other and all that side load is transmitted across the balls to the outer race ball bearings are made for radial loads not thrust loads this will rob a bunch of power and ruin your bearings in short order. So the short answer is to make a spacer that will contact both inner races and still allow the bearing assy. To sit all the way in to its bore
Have fun