Probably a tad between 3,000-4,000 rpm but most likely you'll lose some between idle and 3,000 as well as over 4,000 rpm. You have airflow bottlenecks at the carburetor, intake manifold, exhaust manifolds and exhaust side of the cylinder heads that is going to keep that camshaft from doing much good. An RV or "torque" grind will work much better with a stock engine.
If you're out to improve performance of a stock C-code 289 without spending a lot of money are to home port and port match your cylinder heads, use some non-restrictive exhaust like reproduction HiPo manifolds or tube headers, install an aluminum dual-plane intake like the Weiand Stealth, and a 4V carburetor - the Autolite 4100 is a fine unit. Now, when you add the 268H (after making sure the rest of your valvetrain is up to par), you should get a nice little bump.
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