Cam/vacuum

It just does. Yes it's the cam specs and working on engines with that camshaft before. Leaner mixtures, which is what you want, require more timing to ignite. If you know that retarded timing may cause low vacuum signals why fight reality?

You are not timing a stock engine at this point and using anything related to timing events from the factory is useless/worthless. The factory also had to deal with emissions, which required RETARDED base timing to pass the testing levels. Hemi, six pack, 340's even 318 did not have sufficient base timing. Give the engine what it wants, it will tell you with timing and a vacuum gauge.

Do the test yourself and observe the results as AndyF and I suggested. There is a lot on the internet about the "whys" of this.

Leave the total timing where it is as long as detonation is not present.