I'm a bit rusty on this, but, as I recall, 5 speed spacing was typically 120mm.
126mm started with 6 speed and continued thru 7 speed.
Spreading your dropouts, if 120mm, should not pose a problem.
7 speed should work with your rear derailleur, but you will need a new 7 speed chain.
That may not play too well with your front chainrings and/or derailleur.

I'm sure if I have led you astray, there will be someone along to correct me!
Good luck.