The Effects of Widening Seat/Chainstays first time so bear with me please.
I recently had to replace my rear wheel. Went to LBS and the only options available are 130mm spaced. I have a nice Specialized 1980s frame that measures 126mm. It's a steel frame so squeezing in the new wheel is no problem. However, I'm curious how this will affect the feel of the bike. By essentially widening the "stance" of the bike, will I lose some responsiveness and agility or will it feel the same? Followed up by how will this affect the tension of the shifter cables that travel along the chainstay? Thanks for your help!

PS. If anyone knows where to find a 6 speed 126mm QR rear hub that I can build a wheel around (or a complete wheel) let me know. I've not had much success searching for one. Thanks again!
Not sure what you mean by 'stance' of the bike, but rear wheel will not be moved relative to the bottom bracket by a perceptible amount. No change in handling will result.

Similarly, shift cable will not be affected, but can be adjusted anyway.