Full length headers make the most power across the board, shorties are barely better than manifolds, and mid length are a decent upgrade with lots of clearance. Tri-y headers make excellent torque in a "street" powerband and are great for tow trucks etc.. A header flange needs to be thick so it doesn't warp so try to find ones that are 3/8" thick, no thinner than 5/16". For a 350 you'll want 1.5" primaries for a low performance cam/ heads, 1 5/8" for most setups, and 1 3/4" for a TPI or highly modified 350. For pipes you'll probably want 2 1/4" diameter but 2.5" is the most common and is alright for almost every 350 (just not perfect). Ball and socket header collectors are great- they hardly ever leak and don't require a gasket, 3 bolt flat collectors are most common, 4 bolt versions exist. If using the common 3 bolt type skip right over cheap paper type gaskets and buy the aluminum kind or you'll be replacing them again and again. A crossover pipe like an H or X helps equalize the back pressure on the exhaust banks to promote better/ equal flow. Chambered mufflers hurt flow but sound good, fiberglass packing kills high frequencies.
Many people simply buy a Pypes G Body kit but you can buy a "box of bends" and put together a nice setup with only a couple of hangers. Or skip the kit and just buy the exact pieces you need. You can easily do this for $300, less if buying used, more if getting fancy with stainless. Headers: $125 Mufflers: $50 Leaves $125 for pipes, clamps, and hangers.