Alternatively one use the output side (right) of R10 for SCL and R37 for SDA. Add strain relief or fixation.
You can solder these wires directly to the same SDA/SCL lines on the ESP8266 too.
To test the clock generator is working, open the GBS Control web gui. Go to Preferences and scroll down to Activate FrameTime lock. Press on the FrameTime Lock button and the console will read "Active FrameTime Lock enabled, adjusting external clock gen frequency". Video output should be pristine and free of horizontal tearing when high output resolution presets are used. If however the clock generator is not detected, the console will read "Active FrameTime Lock enabled, disable if display unstable or stays blank!".
If you don't want to place the clock generator to the heat sink, then you can alternatively place it to where the potentiometers used to be. If you do the bridges on the bottom of the GBS board, then this leaves you with some blank space where you can attach the clock generator.
You have to use a small piece of insulating material (I used 2mm and 0.5mm FR4 which is the material circuit boards are made from but without any copper on it) to raise the clock generator up a bit. And preferably use double sided tape for mounting as this can be removed if you have to for any reason. If you remove a little bit of the clock generator board to make it shift closer to pin 40 of the TrueView IC, then the "GND" pad of the clock generator actually matches a GND pad on the GBS board which allows to connect GND by just sticking a wire through both boards. This provides good, low impedance, ground coupling between the two boards.
This is more a technique for advanced builders. If you don't have the right tools, then it is OK if you place the board to the top of the heatsink.