I am having a little trouble getting my idle mixture dialed in on my 1050 Holley marine dominator's.

Where do you hook up the vacuum gauge, I have been trying without one, and with little success.

I drilled and tapped a hole in the manifold just under the spacer plate.

If you have a space plate, drill and tap a port in it. If not, do as stated above.

I have a 1inch spacer, but it is composite, do you think it will still tap ok?

I have a Wilson spacer, but was afraid to drill it. The manifold was easier for me.

You can drill and tap composite no problem. You might want to use a pipe sealer on the fitting.