It's a bbk hi volume part number 1607
This in tank pump is rated as 255 liters per hour(67GPH) at 0psi(free flow)
As pressure goes up flow goes down, I like to use 4GPH loss for each 10psi.
So 40psi system with this pump would be capable of 51GPH
70 PSI system 39GPH
4GPH is just a number I use, calculating pressure vs volume is a formula based on several factors, and there are charts you can look at if you want more exact figures
Yes, the frame rail pump sets the flow, wouldn't matter what pump was in the tank, so I would eliminate that frame rail pump.
Also check the fuel pump voltage, GPH is usually based on 13.5volts(engine running voltage), if voltage drops due to wire connector corrosion flow will drop as well, and it can drop quite fast, i.e. 2 volt drop can cut flow by 1/2