With 120, I generally cold set instead of spreading. And I generally advise against going up more than one 'size', so 120 to 126 or 126 to 130, but not 120 to 130. 10 mm of cold set or spread can cause problems with the brake bridge and/or chainstay bridge if there is one.

Good news is by going to 126 you can do a LOT, like a 7 speed freewheel on your existing wheelset (respaced to 126 and redished), or any mid-late 80's Japanese hubs laced to a modern set of rims on which you can use 7 speed or '8 of 9' or '9 of 10' (look up those terms for a full explanation) on the narrower freehub.