Cyrus,
Mounting the FF sensor between the servo and flow divider is the preferred location. Some mount between the firewall bulkhead fitting and the engine pump with success. Having mounted one on the engine accessory case using the vacuum pump studs I can attest this is not a good place. The vibration proved to be detrimental to the wire pig tail.
My last build I made an aluminum plate that fit between engine mount tubes and adel clamped to the tubes. Then bolted the sender to the plate. It is inline with the vertical path between the throttle body and the flow divider, also protected from engine vibration.