Changed about 15 pumps on these GM's buckets all look the same for the most part just differant height's for differant tanks.
on the stock pump the inlet with the sock is for the motor the other is to cool the pump, the first walbro I did on my old truck I butchered the bucket but it still served its purpose and never starved the the truck of fuel. on my new truck I made a cool spacer for the intake on the pump and it fit just like oem, but not everybody has has a machine shop to use (always make friends with teachers at your local college).
And Zeake is right about the hose too use good high pressure F/I hose and double clamp if you can.