Need Opinions. Have nothing to compare to.

I don't recall off the top of my head what the actual vacuum was reading, but yes, it was pretty steady. I remember thinking it was a little low, but I don't remember if it was around 11 or 13. I want to say it was running around 11 inches because I don't think I would have thought 13 was low. Borderline maybe, but not low. At the time, I wasn't really thinking about specific numbers. I was just tweaking the four mixture screws a little at a time until max vacuum was reached, to make sure they were all set evenly (as per my handy dandy "Tuning Demon Carburetors" book!).

I will run back to the shop and check vacuum tonight. It's about 25 degrees colder today, but I'll let her warm up good and check it quick.

I did find it interesting that the book said to start at 1.5 turns out, but carb was set to 3/4 turns out. I adjusted them out approximately 1/4 turn more (1/8 turn at a time each) before vacuum started to drop off. Reset them back to max vacuum where it started to drop.

Yes,... he clearly said they rebuilt the motor, but it sounded like he had a shop do the rebuild because when he was talking about the engine he specifically said "They did some machine work and put in a bigger cam" not "I" or "We"
Well, You're going to have to do Your own detective work here.
1) As dukeboy_318 said, those are some lame-*** lo-po mills stock, and just putting a set of blue Felpro's on would likely drop the OE squeeze below 8:1. So unless that
things got better slugs or they whacked the 906's a ton, that's where the lazy lies. If they did whack the heads, look for the slightly raised portion of the intake surface to
be almost gone, assuming they cut the intake face to maintain the manifold alignment/height. Do a warm compression check, throttles wide open, see what You've got.