Alex,
It could be a couple of things. Start with the easy stuff..
1. Remove and clean the thermostat ground.
2. Re solder or replace main relay.
3. Remove electrical portion of ignition switch and clean and check solder joints.
The reason the wiring is HOT is because you are running a Walbro 255 which draws more amps than stock fuel pump did. Not enough to blow the 10Amp fuse but close..
How the Main Relay works