How to get the right amount of cable on a handlebar

[Ajax Bay]

The easiest option is to leave all be. The brake and gear cables will function perfectly well. The outers look in good nick though the OP has no idea how long they've been on. Good practice is normally to replace but this context is that stem and bars have been changed rather than normal reasons.
Given the OP's aim/'struggle' I would use @Vaping Joe's ziptie hack right up near the stem (plus electrical tape; tie can be taken off when the OP gets to the final few turns of the bar taping) and take an inch or more (see Ian's advice ^^) off the brake outers.

 

[mickle]

You're over thinking it. From the cables POV the extra few mm of stem = the fewer few mm of bar width. Tape the cables tightly to the middle of the handlebar bend. Wrap the bars. End.

 

[Mr.B-Campag]

Thanks everyone, sounds like it's best to leave as is then! I might try the cable tie close to the stem trick. Fyi I gave the info about the stem being longer as I wondered whether that might counteract the narrower bars enough and it sounds like that's the case.

 

[Ajax Bay]

Ref 'counteract': not significantly. Geometrically it makes far less difference that the narrow bars make: that 43cm > 38cm effectively makes the outers one inch longer (and therefore needing to execute a loop of greater radius to get to the stop on the top tube or the front caliper itself.