"Stock" 350 heads covers alot of ground; from 165 HP smog crap in the mid 70s, to L98 heads and LT-1 heads in 1970, and a whole lot in between....
Chances are though, you have some sort of smoggish, low-compression heads. Your best cam choice would therefore be one with relatively low duration, that would create as much cylinder pressure as possible.
Look at one of the Comp XE series, or others of a similar design with aggressive ramps and high lift, but relatively low duration; and a much bigger exhaust lobe than intake; and 110° or closer lobe centers. 210-216° intake and 220-226° exhaust is plenty. The first 2 that zippy mentioned (the Comp and the Summit) would probably work well, although the next bigger Comp would probably be better.
"So many Mustangs, so little time..." ICON Motorsports