Difference in fuel pumps?

I have a 1986 360 that I am working on, everything is fine except the mechanical fuel pump will not pump fuel. I thought I had a bad fuel pump, but I bench tested it, and it worked fine. I used a flashlight and a mirror to check to see if the pump arm offset washer was in place as I turned the motor over, and it was. I still cannot manage to get the pump to work.
I am using an early 70s model la engine fuel pump, did the pumps change on the later models la engines? I am also using a late model 1986 timing chain cover, is there a difference between the early and late model covers that would cause the pump not to engage the mechanism? Any help would be appreciated.