I think this schematic is probably close to your bikehttp://www.thisoldtractor.com/guzzi007/schematics/1973_V850_USA.gif
You need a pair of relays, I like to use the Bosch cube style because they are period correct and have a mounting tab, mount these in the headlight bucket or close at hand.
Disconnect the green and Green/Black wires from the dimmer switch and put one to the 85 terminal of each relay, the 86 terminals are wired to chassis or black wire. So now the dimmer switch is picking up one or the other relay. Wire the bulb Hi / Lo filament to the 87 terminal of the relay (don't worry which one), all you need now is a source of 12 Volts to the 30 terminal of both relays, this can be the same fuse that feeds to the dimmer switch (Grey/Red I think) or you could run a new supply from the battery.
If the dimmer switch operates the wrong beam swap over the wires on the 87 terminals or the wires on the 85 terminals and it will work correctly.
I like to add a 40 Amp main fuse to my bikes right at the battery, this protects the feed to the ignition switch and Voltage regulator. The fuse also means you can safely tap into the large red wire at the ignition switch, I use that to feed headlight relays by-passing the ignition switch, another source of Voltage loss on an old bike.
The headlight will operate as before only brighter.
You might also consider adding a relay to the horn while you are about it.