Has anyone had success using a vacuum guage to set their timing on a 65 400 stock engine? If so, what should I be looking for in and any other tips good or bad would be helpful. I have heard pros and cons to this so I'm not sure what the truth is.

 

You can do it, but it's more accurate with a timing light.

 

I have done it that way.Advance it to the highest vac, and turn it off and see if it starts easy, if it does not start easy, back it down a little till it starts easy.

 

If Octane is not an issue what do you think the maximum setting with a timing light might be and if I am looking for maximum HP how would I know when I am there? I have heard the only way to get to maximum HP is to Dyno the engine.

 

Harv is on the money on this. One other thing you don't want on the vacuum gauge is the needle oscillating but timing won't cause that.

 

When you say starts easy are you referencing when the engine is hot, cold or both?

 

^x2^
More accurate & my preferred method, as well. Nothing wrong with the Harv and/or Glenn method.
The 1965 Oldsmobile CSM should specify/demonstrate the appropriate timing (and method) for tuning the 400 cid engine. Dwell, Timing & A/F mixture. Or, it should be on a label on the fan shroud (if the label is there). If you're after the absolute exact (documented) top HP for the engine then you'll have to use a dyno.

 

What I mean by starting easy, is if you have to much timing, it will crank hard, like the starter can't turn the motor over, and, may backfire or kick back.

 

It all depends on how much mechanical advance is built into your distributor. Read the links below, although they ref Chevy engines, it actually applies to all v8's.
Engine_Timing.pdf (corvette-restoration.com)

TIMING AND VACUUM ADVANCE 101 (rockridgefarm.com)