Electric or Mech Fuel Pump? For a carb...

You can swap the cover and still not use a mechanical (I don't)
I like that the carb has fuel the second I turn the key for on, and if you ever inadvertently run out of gas you don't have to crank the crap out of it to get fuel back up there.
The main reason to use or not use the Magnum front cover and water pump is to keep the serpentine belt system.
If you put the LA style covers and water pump on the Magnum you run a regular V belt system.
There are other cam and mechanical pump details such as needing the eccentric on the cam for the fuel pump, and those can be gotten and added (Mag cams don't have them originally)

Recirculating (or return) fuel systems are a function of EFI normally, but we can add the same type system in our cars with carburetors to keep cooler fuel available to the carb and to solve vapor lock problems from that hot fuel.