Go with the stereo...and the pulleys! These S/N-95's come with a high power 200 amp alternator, unlike the old fox body's which came with a 130 amp alternator.
I have the Mach 460, which cranks out 200 Watts of power, plus a 700 Watt amp pushing my three JL Audio 10"s. I've got a set of March U/D pulley's that I installed and have had no charging problems what so ever. I even use the A/C sometimes. While there is a definite power drop when you kick on the A/C, there is no power loss from using the stereo.
However, if you do experience any charging or draining problems, simply install a Capacitor in your stereo system. I have a 1 Farad Cap that I installed.
The standard for Caps is 1 Farad per 1000 Watts of power, so for your system, I would say use a .25 Farad cap, if they make it. I know there is a .5 Farad cap, and it doesn't hurt to go bigger.
Trust me, Stereo + Pulley's = No Problems!
Check out my system by following the link in my signature.
Good Luck!
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T-Bones '96GT
'96 GT 5-speed
Roush Cold Air Kit w/ K&N, 80MM Mass Air, Mac Cat-Back, Mac Pro Chamber H-Pipe, FMS 3.73's, FMS 70MM TB, March U/D Pulleys.