On my 1997 6.5 there is a small flat spot just bellow the turbo flange ( pre turbo ) is this where I need to drill and what size is the most common. I was going to drill and tap it and put a plug in it until that time.

that's where i drilled and tapped mine. I can't remember the size of the tap but it was an 11/32 drill bit that was used to make the hole.

I was able to drill and tap that location without removing the manifold or the inner fender. Just pull off the rubber water deflector that's held on with snap clips, and cut the wheel one way. Make sure to drill with the engine idling, so the metal chips are blown out instead of falling into the manifold and goign into the turbo.

There was a thread a while back with pics. Try a search. OMG I just turned into a search nazi. I'm sorry.