You won't find the space you need to mount the hum pots through the chassis plate in a reasonable manner, I would just solder them to the tube pins. This is part that is only adjusted when you swap tubes, which is pretty infrequent with 2A3's.
Once the amplifier is 100% functional, you can stand up the driver C4S board and silicone it to the top plate with two dots of silicone.
We do now use standoffs and hardware, but this kit is not setup for that.