On one engine we drilled and tapped the fuel pump block-off plate and read the crankcase vacuum from there. On another engine I drilled and tapped the valley cover portion of the intake manifold as you mentioned. I had to come back and install a valve cover breather baffle under that hole because it was getting oil splash from the crankcase. So be forewarned that you may need to install some kind of baffle. On other engines I have seen a bung welded on the cast aluminum valve cover and they were reading vacuum from there. Again it was shielded.
I really don't expect you'll see any vacuum with a three stage (two scavenge)dry sump pump. Maybe a little, but it generally takes a four stage in order to pull enough to see any vacuum. With a four stage you can expect 10-12". Like I said earlier, you need to seal the engine up tight. Generally the thin sheet alum. valve covers, (like Unique Products) are out of the question. The Jeff Johnson style that has the heavy billet style of gasket rail will work, or the sturdy cast alum. valve covers.
Hope this helps,
Charlie
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