318 Cam

On a stock 318, the 260 will be a happier combo. The 268 really likes more then 9.0:1 compression. So I would say no, you won't gain anything, and may in fact lose some of your low end. The 268 really likes a bit more stall speed in the converter as well. In any event keep a split-pattern cam, the added exhaust duration will help with your system.

As for the a/c, the compressor is nothing more than an idler pulley with the system in the OFF mode. For maximum performance, turn the a/c off.