The swing arm slots are what align the axle. If it's just forward on one side, then the chain/axle is just not adjusted right. With the wheel installed loosen all the bolts on the brake back plate (3 including the axle), and shift the wheel/tire assy until it is square. Measuring between the axle and swing arm pivot is a good way to to do it although most of us use a long straight edge from the back tire to the front tire so if any thing is out of square the wheels won't be. You are probably setting the chain adjusting snails where they are in the same "Slot". That is not correct, those snails differ from one side to the other so you cannot adjust both the same and have correct wheel alignment. When you're done aligning the wheel and have the chain adjusted, about 1-1/2" slack sitting on the center stand, and the snail in the proper spot, crank the brake adjuster nut down until the rear wheel is locked up. Tighten the inside nut, the big one, that holds the brake backing plate, then the axle nut, then the brake shoe pivot nut, the little one. You can loosen the brake adjuster now and that should get you all lined and and the brake properly set up.
Bare