The extra load on the alternator when the fan cuts in is probably causing the idle speed to drop too low. A work-around might be to increase the idle speed, but that will increase overall fuel consumption and, if it's an auto, increase the jolt when engaging gear.
EFI models use a solenoid to increase the idle speed when ancilliaries such as fan, aircon etc cut in. I've no experience with Holleys; maybe something similar already exists. Devising one from scratch could take quite a bit of work.
Try asking one of the RV8 specialists for advice, e.g. RPI Engineering.