well first off, changing the gage of the wire to a bigger gage will only allow more amperage to go through the circuit...fuse links, fuses, and circuit breakers have one purpose in life, to protect the wiring-not the components the wiring runs to. if you increase the gage, you're only upping the potential to burn up more wiring...
i'm betting you fried more than just one wire. and if grounds aren't properly done (corroded, not well fastened), then too much amperage will be drawn through whatever circuit you're energizing, thus burning up wires and possibly components.
when you were doing this swapover, did you change anything in the electrical system which could do this? also, the computer only provides a short to ground to control all the "actuators" (injectors, egr valve, fuel pump, etc). did you change any components that may now be drawing more amperage than what they should? did you piggyback (splice) anything off the computer's wiring-this is a no-no btw-this will cause too much amps to be drawn from that circuit
