First, see Jack's build: https://www.buildyourcnc.com/JackRehak.aspx
The difference between mounting the electronics in the gantry is that the connection from the computer (signal level) is long. The alternative, where the electronics is near the computer, the motor cables are long (to reach from the drivers to the motors).
Mounting the breakout board, drivers and power supply within the gantry will cause you to have a long signal connection from the computer to the breakout board. That is fine as long as the cable is long enough, possibly shielded, and can carry the signal at long cable lengths. Make sure to have the cable well away from the motor cables as these will cause quite a bit of interference.
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Provided they will fit and I install a ventilation/cooling fan, is there any reason why I should NOT mount the electronics (breakout board, stepper drivers and power supply) inside the gantry box in my BlackToe 2x4?