Simply run a thick wire from the big stud on the alternator to the big stud on your starter relay. The factory wire is 12 awg (my 60's Mopars) so use thicker (at least 8 awg). Smart to run it in a sheath (SPC Tech PVC tubing or split loom).
If your battery is closer or easier, run the wire to BATT+ instead. Either way, you still have the factory wire (12 awg my cars) between BATT+ and starter relay, so smart to up-size that too.
Other ideas - put as many under-hood loads as possible on relays (powered from BATT+ via fuse). Many put headlamps on relays. Mandatory for an electric radiator fan. The factory already put the horn and starter on relays. If adding big cabin loads (high amp stereo?), you might do the "fleet bypass" where you hard-wire the ALT and BATT circuits rather than rely on the spade terminals in the bulkhead connector (commonly melt, unless 1963 or 1965).