how much HP do Shorty headers give you?
Im going to put some on and some of my friends say that I will need to pull my engine to install them. I have A 87 iroc L98 350 5speed With a flowmaster Y-pipe, Im putting true duals on it when I do the headers, and I have the cat already cut out.
Any way, how much HP will it give me and do I need to lift the engine to put them in?

 

It really depends upon the engine onto which they're being installed. Headers on a highly modified (cam, porting, inductions improvements, etc.) will make more of a power difference over stock manifolds than the same headers would on an unmodified factory engine. But, 10% is fairly typical, more with more extensive mods.

If you're eliminating the cat & y-pipe, you might as well go with long tube headers. The gain will be even more on a modified engine.

You don't need to "pull" the engine to install them, although putting them on an already removed engine is easier than putting them on with the engine in. You may have to disconnect a motor mount and lift the engine up a little, but you won't have to completely remove it.