The way I read the diagram, +12V is already through a relay at the underhood fuse box, so what you need to improve is ground. (Though it wouldn't hurt to upgrade the wires from the relay to the headlight fuses, and then from the fuses to the actual lights to 10AWG.)
According to the
TEWD, power goes from the battery, to the main fuseable link (white) to the underhood fuse box. There it connects to the headlight relay. From the relay, a red wire goes to each headlight fuse, and from the fuses, one red, and one red/white wire goes to each headlight. Not much room for improvement there. Ground on the other hand goes through a bunch of connectors, passes through the firewall, and goes to the combination switch.
So a simple solution is to use a single 30A relay (It's a pair of 15A fuses, so there will never be more than 30A in the circuit without blowing fuses)
Pin 30 connected to a nice solid ground
Pin 87A(NC) to both low beam grounds (R-L connected to lamp)
Pin 87(NO) to both high beam grounds (R-B connected to lamp)
Pin 85 to the (R-B connected to combination switch)
Pin 86 to the R wire between the headlight relay and the fuses, assuming that your relay is at or very near the underhood fuse box. I'd probably add a very small fuse on this line for safety.
This does change the way that the circuit works, in that the combination switch works as a slight "make before break" between the two circuits, so when you switch to high beams, or flash, there is a time while both headlight elements have power. Wiring this way will remove that functionality, since the relay is a "break before make" switch, so the low beam will lose power before the high beam receives power, and vise versa. You may perceive a slight "blink" in your lighting while switching from high beams to low beams.
Alternately, wire as above, but disconnect 87A. Instead, wire in a second low beam relay, substituting the R-L wires from the headlights and combination switch. This will keep the factory functionality (Assuming that your relays are fast enough. They won't be instantaneous, so it's not guaranteed to eliminate a blink when switching between low and high beams. (This really is being anal, but if you're worried about WIRE LENGTHS, it bears mention.)
It would probably actually be better to use a pair of high power/low voltage NPN transistor and a resistor in place of the relays, which would be much faster (delay < 1ms) and deal with the power adequately, however the semiconductor will have a nonzero resistance, though likely lower than that of the stock setup.
