Thinking out loud, wouldn't you have been able to use a double alt pulley with a extra long belt on the inside to go around crank, wp, alt, and a/c? With a standard length on the second sheave. Tension the alt and then tension the a/c compressor. Or even two long belts? Maybe your a/c pulleys don't line up on both sheaves, they must not. Its hard to tell in the photo.
The setup you made will most likely work great (looks good too) but it is much harder on v belts wrapping on the back side. But is done all the time to ensure enough belt wrap.
Lenz is correct, mower belts are some of the best made. Kevlar wrap makes a big difference in life.