* Cylinder Heads: Different heads (stock, aftermarket, etc.) drastically change airflow and thus horsepower.
* Camshaft: The camshaft profile significantly impacts power and the engine's powerband.
* Intake Manifold: The intake manifold's design affects how well the engine breathes.
* Exhaust System: A restrictive exhaust system will rob horsepower.
* Compression Ratio: A higher compression ratio generally increases power but requires higher-octane fuel.
* Internal Components: The connecting rods, pistons, and crankshaft all play a role.
* Carburetion/Fuel Injection: The type and tuning of the fuel delivery system is critical.
A stock 351 Windsor produces roughly 260-300 horsepower depending on the year and application. Boring it .030" over slightly increases displacement (and therefore *potential* horsepower), but without knowing the specifics of the other components, any horsepower estimate would be pure speculation. It could easily range from slightly more than stock to significantly more with performance modifications. You might see a 10-20 horsepower increase with a simple .030" overbore and nothing else changed, but substantial gains are possible with added modifications.
