At idle how much oil pressure should a Chevy 350 produce?

You should have about 10 PSI per 1000 RPM; a touch higher at low RPM and a tough lower at high RPM.

If you car idles at around 800 RPM, 10-15 PSI is good.

The 250 - 275 value mentioned by another poster would likely blow oil past all of your seals if it were possible ot get oil preasure that high.