I haven't had time to look in detail, but it appears that your biggest problem is your input and output jacks are soldered directly to the PCB.
You have a couple of options:
1 - pull the jacks off the PCB, use chassis mount jacks instead, and relocate the PCB. This would allow you to proceed with the mod as usual
2 - identify the pins on the jacks that you need to interrupt with the 3PDP switch (i.e.the input and output), pull the jacks off of the board, cut those pins off so they don't go through the PCB holes. Solder wires to the remains of the pins on the jacks, and the underside of the PCB where they would normally make contact. put the jacks back on the board, then proceed with the mod as usual. BTW, this would only work for a single layer board - all bets are off for a two layer board.
3 - same as #2, except cut the traces on the PCB instead of cutting the pins. Again, things get tricky with a multi-layer board.
Got any pics of the board layout?