[DonC]

Mounting location for the bung will depend on how your exhaust system is configured.
As a rule of thumb you want it as close as possible to the exhaust ports at a point where the exhaust for all four ports is passing.
If you're running individual header pipes to a collector put it in the system at the collector or somewhere down stream. In some systems this will put it a long way from the exhaust ports. I believe the general consensus is within about 18" or closer to the exhaust ports for best results.
DonC