If you look at your swing arm and measure the thickness of the axle plates you will find them about 3/8 inch thick. On your hardtail the axle plates are about 1/4 inch thick.
They make the axle plates 1/4 inch thick becuase the early, 70-73 bikes had 1/4 inch thick axle plates. If they made them thicker the early axles would be to short.
Just get a hand full of 3/4 inch washers. Put one under the head of the axle and one under the nut. Tighten it down. If it tightens down ok then run it. If it doesn't tighten down ok, add another washer under the head and nut.
Leo