I have a 84 GMC Sierra with a 6.2l motor i bought it for 300 bucks didn't run but with a little tinkering i got it running and the water in fuel light came on so i drained all the fuel and replaced the lines and bled them after and the light still comes on what else could it be?

 

Water in fuel sensor itself could be shot

 

You also need to get the fuel filter housing drained-there is a brass tap near the thermostat housing that is meant to be opened when the engine is idling to drain that water.

Beware of moving parts.