20 inches at idle probably means you have your idle set too high. 15" or so is plenty, most shoot for 16-18", I'm happy with 5"- as vacuum in itself means very little.
To get it properly tuned you can use a vacuum gauge and plug reading, but the best way by a long shot with with a A/F gauge.
For this engine box stock setting won't be far off at all.
As for the exhaust question, widen the LSA by 2 and add 5-10 degrees to the exhaust duration.
No manifold is a good manifold though- the best though still lousy would be the LT1 manifolds you'd be surprised the difference a good exhaust can make. night and day.