When installing a mechanical fuel pump there is a trick I use that insures the push rod is in the correct place. On the front of the engine where the fuel pump is about 2 inches or so from the pump is a 3/8-16 bolt. Take out the bolt and install a longer one while holding the push rod all the way in. This should hold the rod in place while you install the pump. Make sure to replace the bolt with the smaller bolt so you don't wipe out the cam lobe or have an oil leak from having no bolt in the hole. I'm not 100% sure but aren't the fuel and vent lines different sizes?