[RichardJ]

Using a 10 gauge wire for the HEI IS THE EXACT OPPOSITE of a fusible link. A fusible link is, by design, the weakest link in an electrical circuit. A fusible link is designed to FAIL first and protect the rest of the circuit.

A 10 gauge wire for the HEI becomes the strongest link in the circuit.

The 12 gauge wire from your fuse box to the ignition switch, the ignition switch, the 12G wire from the ign sw to the bulk head connector and the 12g wire to the battery, all become more susceptible to excessive current than your 10 gauge HEI wire.
The truck could burn to the ground and your precious 10 gauge wire would remain unscathed.

The original, factory HEI wire connecting the HEI to the bulkhead connector was 14 gauge wire. This 14 gauge wire had a very unusually extra thick insulation. It might be the extra thick insulation that has led many to believe it is a larger wire.

As dmjlamber pointed out, the factory HEI uses less than 2 Amps. I've measured it. Apparently djmlambert has also.
OK, you don't believe me. Would you believe General Motors.
The image below is from the 1975 Chevy Truck page in the GM Heritage Archive.

Ignition - 2 Amps, The same as the brake lights, Half is much as the Horn. Although these items draw current for short time periods.

A 10 amp fuse will allow the HEI to completely melt, before the fuse blows.