Useless mod, worth maybe 2-3 HP. The factory manifolds are not really that restrictive. LT's would be your best bet because they are designed to use the exhaust pulses to help pull exhaust gas out of the engine (scavenging effect). For the effort involved to install shorties on a modular engine, you're better off spending your time/money elsewhere. If your hell bent on buying headers, save up for some LT's.
not sure what other mods you have, but 4.10's, UDP's, or the popular PI intake/cam swap would likely bring much more of a smile to your face than the headers.
i got udp bbk plenum jlt intake ram hdx clutch exaust 4.10s short throw and a 75 shot the kid just told me 100 bucks for them and i thought that was cheap but i didnt no if they were any good and worth my time puting on