A 126 wheel in a 135 frame is probably a bit too far to just spring the frame on a long term basis. The dropouts can fail where clamping the wheel forces them from slightly angled in to straight if you do it too long.
There's also no point in cold setting a new frame to an old wheel size.
Assuming the wheel is QR, you'll need a longer 135mm axle and some spacers. The axle end has to come to just short of the outside face of the dropout. Put most of the spacers at the non-gear side. If you get the smallest sprocket the same distance from the chainstay as with the new wheel this will minimise fiddling with the gear indexing when you swap wheels. This will leave the cassette in the right place relative to the rear mech, but will mean that the wheel has to be redished.
Slack off all the gearside spokes half a turn, and tighten the non-gearside spokes a turn, then assess how much the rim has moved and how much further it still has to move. Repeat as required, reducing the amount of movement as the centre is approached. Finish off by truing the wheel properly (see Sheldon or wherever for truing advice), and adjusting your gear indexing
There's no need to be too exact when getting the rim central. So long as the brakes are happy, within a couple of mm is OK.
Afterthought:
If the old wheel is 126mm and the new wheel is 135mm, will a new cassette the same as the one fitted to the new wheel fit on the old hub? A 135 hub will have an 8/9/10 freehub body on it, and my memory is that 126 wheels were 6 or 7.
If the cassette off the new wheel doesn't fit the old wheel, then you'd be looking at the possibility of replacing a 6/7 freehub body with an 8/9/10 one (if they fit). This would have to be done before deciding on spacer location and wheel redishing.
PS
The palindrome of Bolton is not notloB. A palindrome is a word or words that read the same backwards as forwards. BoltonnotloB would be a palindrome if it were a word or name, but it isn't.
The only UK place with a palindromic name I can think of is Glenelg.
The classic multiword palindrome is "Able was I ere I saw Elba" (Napoleon was exiled there)