Intake ports (PI heads left.....NPI heads right)
Stock NPI GT's will put down around 180-ish rwhp. With a full PI swap (PI heads, PI cams, PI intake) you could see around a 30-40rwhp gain. The PI heads alone are good for maybe +10-12, PI are good for about +15rwhp, and the PI intake usually sees about another +15rwhp give or take. Seeing as you could find these parts used for pretty darn cheap, a PI swap is one of the cheapest and best "bang-for-the-buck" mods for an NPI car.
It depends on the grind. Stage 1 cams, no matter who they are from, are usually pretty mild and don't require any valve train modifications. Stage 2 grinds are basically 50-50. Some can get by on the stock valve train, others can not. Stage 3 grinds will most likely require an upgraded valve train. The manufacturer of the cam will tell you whether any valve train mods are required. Look into Modular Headshop, HiTech Motorsports, and Comp for popular mustang cams.