Nominal 6-speed spacing is 126mm though one hub or another came in at 127mm. Looks like you are at 127 to 128 and that's ok for either 126 or 130. A steel frame will adapt readily. Checking that the dropouts are straight and parallel is another matter. There is a tool for that, but I've never seen one in the flesh.

Standard way to quickly gauge the straightness of the frame is to attach a string from one dropout to the other passing it around the head tube. Then, measure the distance from the string to the seat tube -- should be very close to the same on each side. Also, when you insert a well-made wheel in the dropouts and center it at the chain stays, it should be centered at the seat stays as well.