The kit looks good to me. The spring is good too, I read that on the old springs you could stretch them out to make the pump work better.So the longer spring will make it work better.
Take a look at the second photo, that pump halve ( that the spring goes in) has discoloration in 4 spots between the screw holes. That is just where the pump I just rebuilt was warped. It looked just like that too! Put a square across all 4 sides and see if there is a gap in the dark stained locations. What happens is it will suck in air at those points and not allow the diaphragm to build the pressure needed to pump the fuel.
Let me know what you find. And good luck.