[olddog]

I assume flat tappet mechanical lifters. I have very similar cam specs in a SB only it is hydraulic roller lifters. I can idle it down to 800, but I have EFI. If memory serves it idles about 11"Hg. I would expect more vacuum out of the larger displacement.

From everything you have said, I think it would be worth checking the valve lash.

Take everything I say with a grain of salt. I'm just an armchair enthusiast that owned a 390 FE in 1975. In short, I don't know much.