When the locknut faces on the wheel spindle are 126mm and the dropout faces are 120mm, you will need to pull the stays open manualy to get the wheel in.
By doing so, the dropout faces become non parallel. When you close the QR, the pressure squeezes the dropout faces ( not parallel ) onto the locknut faces ( parallel ). One of the two has to bend.
Be aware it will probably be the spindle that will bend when the locknut faces are squeezed out of parallel to match the dropouts.
If you intend to use the 126mm wheel in that frame ALWAYS, do some mechanical adjustment ( bend ) of the dropouts. Risky, because you will also be bending the paint. Also risky because if one pair of seat and chain stays bends easier, the dropouts will no longer be eqi-distant from the frame's centre line.
When I 'upped' from 5 to 6 on my Pug, LA Cycles in Coventry manipulated the frame to suit the wider wheel. They have one of Dawes' old jigs, so it is now quite precise and trustworthy.
The chrome didn't suffer either
.