The 99 will have vortec heads, and they are a good flowing head. When it comes to heads and cams, This is the most effective up grade for performance that you can do to any 350. Aluminum heads will flow better than stock. When it comes to heads, you get what you pay for. Good aluminum heads are not cheap. Vortec heads can be modified to flow very well, and also to except a larger cam. The only issue here is that the OE vortec heads have a thin casting are are prone to cracking issues, and porting and polishing makes this worse. You can buy a set of aftermarket vortec heads that have a heaver casting, and can be ported and polished, and will hold up better to this than the OE heads will. In simple terms, an engine is just an air compressor, the more air you can get into, and out of it, the more power it will make.