Can a (relative) beginner adjust spacing on a 501 cromo road frame? Have acquired a cheap old Peugeot road bike from ebay; nice cromo frame, but it needs a new rear wheel and unfortunately I didn't check the dropout size before buying.

It has an OLD of ~120mm, but I am finding it very hard to source any new or second hand wheels that would fit that, and most of those that do seem designed for fixed sprockets. There are plenty of seemingly 'cheap and cheerful' wheels with 135mm hubs that would otherwise be perfect.

Sheldon Brown's article on frame cold-setting makes it sound easy, but I'm worried it'll be hard to adjust each side to be exactly 7.5mm further out and keep the wheel central, even using the string method. 

Has anyone got practical experience of this? In addition, will there be any other effects on the running of the bike I've missed?
If you're going to be respacing, I'd suggest an alternative method from sheldon's 2x4 method. Use a threaded rod with washers and nuts. It's far more controlled and easier to keep your frame aligned, plus you can keep it in the stand as you work. 

See here 


If you need to adjust the dropout alignment, you can adjust thusly