Where is mechanical fuel pump on 1984 s10 2.58?

On a 1984 S10 with the 2.5L (not 2.58L, there's no such engine designation in that vehicle) engine, the mechanical fuel pump is located on the side of the engine block, driven directly by the camshaft. It's usually near the distributor, but you'll have to look closely at the block itself to find it. It's a fairly small, cylindrical component. A picture or diagram of the engine will be extremely helpful in locating it. Consult a repair manual for your specific year and engine for a clear visual.