Here is information and a couple of diagrams that answer the question about wiring up the FX loop - Provided by Stephen (in lightening fast response time, as usual!).
Disconnect wire from treble to master volume.
From treble to SEND /IN on FX. From RETURN/OUT on FX to Volume pot.
Connect GROUND on FX to Master Volume ground.
If you have RG174U coax, connect the core wires as above. Connect both shields to GROUND and ground as above.
Hopefully, you didn’t affect the board with your surgery!
The location is not ideal because it us so close to AC power and the power transformer. Curious to know how well it works!! My choice would be to put it at the opposite end, but your solution may work. You may need coax to keep the noise down.
One other thing though, you will need to connect the FX board B+ to a fixed voltage. So connect it here.
FX Wiring - B+ Location
https://photos.app.goo.gl/MGUAMjgxyFuQhe3T8FX Wiring “In and Out”
https://photos.app.goo.gl/1B3Zy3vKaDPvRoSw6