You would notice a power gain if your stock fan was a mechanical fan, but seeing as most RBV's used a clutch fan set-up you prolly won't notice any power gains at all.
Go to your auto parts store and ask for an electric thermostat fan relay kit, shouldn't be much more than 30 bucks, it will run the fan on low speed for sure as that's what I used on mine. This elec therm is an automatic on/off type system, no need to manually operate the fan with this set-up for most DD and such. It just turns itself on or off automatically to the set temps of it's own thermostat, mine is a 180 therm.
I only takes that 70amps to start the fan from a stop on high speed, not low speed, low speed can use 20 amps easily. And actually mine only draws 59 amps from a stop to start the high speed circuit.
I wired mine up to an over ride switch so I can turn the fan off if need be. The switch I used was a 2 way separate power switch. Toggle position in the center is off, up is one circuit and down is the other separate circuit. I picked up a 70A mega fuse, some 8 gauge wire and a starter/winch solenoid and a 1 foot piece of battery cable with the brass ends that fit over threaded studs. All this was for the high speed circuit. I wired it all up so that the high speed circuit is switched on by the same toggle switch that turns off the low speed fan circuit.
When my engine temps get too hot for the fans low speed to handle, I flip the switch to actuate the high speed fan circuit, which turns off the low speed circuit as I switch it over to the high speed circuit. Since the fan is already spinning it also doesn't take the full 60A to fire the high speed circuit up. But in the event that I want the high speed circuit to run from the fan stopped, the 70A mega fuse can handle that duty easily. I also wired the high speed circuit live so that I can run it with the ign. key off to cool the engine after shutting it off if I ever need to.
So far it has worked quite well. I have only been running it this way for a couple months maybe, but it's working like a charm so far. The electric thermostat I have had in there for a few years now with no problems other than when I push the engine really hard for extended periods the low fan speed doesn't keep the engine coolant cool enough, but that high speed sure does.