What I also did was to wind a stout rope tightly around the little "bridge" between the chain stays near the seat tube
Steve Snowling, a pro mechanic wrote a rather nice book including just this issue. i must admit to always using his idea. Grip both drop outs , one in either hand at chest level or what ever. Pull gently. The steel will spring a touch so you may have to over pull and few mil. You will apply the same force both sides unless you have a duff arm.
If the frame was an ancient single speeder it just might have been 120mm so you would need to go 130 for a modern road wheel. 135 for modern MTB. If it was a 5, 6 or 7 speeder then it was 126 mm road. . Theorectically the drop outs now are not exactly parallel but you won't notice this in practice.
Easy job done millions of times without the slightest worry. unfortunately we are not allowed to have that concept nowadays