I would run with the entire assembly and return the AC Delco pump. The Vortec gen I small block pump is able to supply the 50+ pounds of pressure needed to run the LS engine. I used a Delphi pump on my in tank assembly simply because I bought a used assembly off Ebay. I didn't want to chance using the used pump. Installed the new pump with the used sending unit. Truck starts immediately and runs great. Plus, my fuel guage now works when it didn't before.
'93 Chevy 1500 4X4 ECLB with a '99 Silverado LQ4 6.0 and '99 Escalade 4L60E. SOLD IT!!
New toy: 2001 2500 HD 4X4 Extended Cab Long Bed
John