how high will a 273 rev.

Bill, I think 6000 is a reasonable top end RPM for a stock-ish small block. A lot depends on your valve springs, cam, intake, and ignition systems. If you're running old stock valve springs, then you may be getting into valve float or coil bind by then. You can usually tell when the power starts to flatten out that you've reached the maximum limit of one of the above mentioned parts.