A heavier gage wire works the same as a lighter gage wire it is just able to carry more current over any given distance. The longer the given distance (say a battery in the trunk), requires an increase in the wire gage to counter the added resistance of the longer wire.
Only down side is because a heavier gage wire is made of expensive copper, it costs more per foot than a lighter gage wire, and as I said above it does add weight.
An HEI distributor doesn't need any wires to be run from the starter solenoid. A heavier gage wire from the fuse box to the ignition key is a plus, but you are still relying upon the fire wall forward stock wiring to power the fuse box. So you don't gain anything until that feed wire is upgraded.