12 in is fine just make sure you use a heavy gas line type hose, that way the hose does not flex with the pulses of the motor, meaning the pump will get a stronger signal.
When you mount the pump you really want to utilize gravity if you can, the pumps will suck fuel no matter where you mount it. But the best setup will have it mounted below the pet**** or at least the bottom of the tank and above the carb for the fuel line and above the vacuum port.
If the pump is mounted below the vacuum port then there is some chance of small amounts of oil building up in the pump hurting the performance. If you did a yearly cleaning of the pump then you would be fine with the amount of possible oil buildup
I always drill and tap the reed cage, I dont like disruptions to airflow to the transfers, I drill centered to the side of the V-force reeds, and if the reed cage is very close you can clearance the reed cage some and I always grind down the threads sticking into the reed area of the cylinder. I also take a countersink bit and open the very end of the brass fitting.
There is not allot of flow in and out of the fitting so do not get to worried having allot of open area on the end of the fitting