The ventral fin was fitted when the B707-100 had the fan engines fitted to make them B707-120 series. They also made the aircraft "geometry limited" as a bonus reducing some margins for the Vr and Vlo speeds. It had nothing to do with the British ARB. There are many other sins to be laid at their door but not this.
It wasn't just the RR Conways which required the ventral fin and rudder power boost, actually the RR engines were little more than a side show given the few that actually flew. It was the P&W JT3D which went from ~12,000lb to ~17,000lb per engine which gave this mod its impetus.
The glove panels screwed to the B707-100 wings to reprofile them for the higher power JT3D-3B engines and hence higher cruise speeds were a PITA for the apprentices who had to screw them onto the wings. Every screw had to be measured lest you screw an over length one in and break the fuel tank seal with resultant tank entry etc, etc.