Hello

This is what you need to put into your CFG.

[General]
performance=
atc_type=BOEING
atc_model=B703
Category=airplane

It should be marked as B703 as the ICAO Code for this type of aircraft, all variants from 707 Captain actually, that way it will identify you as a Boeing 707 Heavy. 707 is the type rating necessary to fly the craft not the ICAO you use to file a flight plan and used by ATC also to identify the Wake turbulence and maneuvering profiles.