Although a HEI originally used a very large supply wire, that was not due to the load required.
A HEI actually only draws less than an amp.
As Tom Langdon, who was part of the team designing the HEI for Chevrolet explained it, the heavy wire had to do with the energy field around the wire being lessened with the larger wire, or something to that effect (it has been many years since he explained it to me).
I have converted many of these with stock wiring and switch with no problems.
