Even if you *could* somehow make them fit, the horsepower increase wouldn't be a simple calculation. The outcome depends heavily on many factors including:
* The specific Corvette heads used: Different Corvette years and models had different heads with varying port sizes, chamber volumes, and flow characteristics. A set of heads from a late-model high-performance Corvette would offer significantly more potential than those from an earlier, lower-output model.
* The rest of the engine's components: The camshaft, intake manifold, exhaust system, and other parts must be compatible and appropriately sized to work with the higher-flowing heads. Using Corvette heads on a stock 305 with a small cam and restrictive exhaust will severely limit the performance gains.
* Quality of the modifications: Proper machining, porting, and valve job are crucial for realizing the full potential of the heads. Poorly executed modifications could even *reduce* performance.
Hypothetical HP Increase (with significant modifications):
If everything were perfectly matched and meticulously modified, you *might* see a gain of 50-100 horsepower, potentially more depending on the Corvette heads used and the level of supporting modifications. However, a realistic expectation would be considerably less if certain parts are not changed to maximize the flow characteristics of the heads.
In short: While it's theoretically *possible* to adapt Corvette heads, it's impractical and expensive. The horsepower gains are highly variable and depend heavily on the entire engine build. It's usually far more efficient and cost-effective to focus on other upgrades for a 305, such as a better camshaft, intake manifold, and exhaust system.
