Electronic fuel pump question

[Pano]

1987 gmc v2500 5.7 tbi dual tanks with pumps in the tanks. I finally put my new passenger tank on after looking at it in the garage for 8 yrs. I reused the old pump which worked fine when I took it out.

When I switch tanks I don't hear anything. I will double check the connections first but my question is if I buy a new pump do I buy the whole assembly or just the pump and mount it to the original assembly? Thanks
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