Body and frame shops have tools to widen and straighten the shock towers and permit the monte carlo bar to be installed. If its far off its best to just take it to a professional shop and let 'em do it. They can also advise you if more work is needed. In a lot of cars, the frame rails have begun to deteriorate from corrosion permitting the shock towers to start their melt...
I'd guess that my shock towers were somewhere between 1/8" and 1/4" too close together. The interesting thing is, the bar is completely straight now and even closer to the air cleaner. I'm not sure how long it took to spring the shock towers back to their proper position, but it did it.
Good Luck with your installation. Try jacking it up under the engine crossmember, let it sit overnight, and see if that allows the fenders to move apart a bit.
i posted some information earlier about this install. i used a contraption consisting of a bar of a jack and a coil spring compressor tool to spread the towers apart. do a search for "coil spring bar sloan" and you might find it.