You need enough thread for the upper headset cup to thread tight before running out of thread. Then you can add spacers to the top nut.

However that's only half the story. The stem must be inserted deep enough that the bottom is about 1" or so below the upper HS bearing. (and beyond the last thread) Otherwise there's too much cantilever stress and you risk the the steerer snapping at the headset.

You might be better off finding a proper length fork, or threading and cutting this down to work correctly, or possibly going threadless, depending on the fork length.

BTW- if you have a slight buckle in the top tube, odds are there's a more serious buckle in the downtube. Make sure to check that. If ti's a steel frame, it'll probably be OK to ride (leaving that judgement to the builder who saw it), but be sure to check both top and down tubes for signs of crack propagation at the sides of the buckle. The first sign will probably be some visible cracks in the paint.

Also note that small cracks in the paint at buckles are often points of entry for water, and the tube will form hairline rust patterns, accelerating the demise of the tube. Keep this in mind if this is an all weather bike.