For what my 1 cent is worth I think the cam you are looking for is the GM "Ramjet" p/n 14097395 cam. Moderate lift and duration is supposed to wake up the stock 350 and still play nice with your oe heads and tune. Haven't used one myself yet, but everything I've read about it (and that is LOTS) makes me think it is something you ought to look in to, if you want good low to mid rpm grunt. Here's a good link to get you started:
http://67-72chevytrucks.com/vboard/s...d.php?t=255659
I bought two of 'em a few years ago just because they were cheap (for hr cams) at $140 for the pair shipped, off of ebay back when guys were replacing them in Ramjet and HT383 crate motors with stupid thumpy cams. My brother needed one and I'm planning to use the other in a 336 (3.75 stroked .040 over 305 block) with L98 aluminum heads. Going in a '97 Jimmy.
My guess is that the ZZ4, while a really good cam (used a couple of 'em) for appropriate use, just might be a bit more (208/221@.050 & .474/.510) than your heads and tune will handle.