67mustang302

GT40 heads basically have the exhaust bump for air injection removed, and are slightly cleaner ports with a minute design difference. There may be other minor differences as well, but they're basically a "cleaner" 351W head. The GT40p's have a revised exhaust port, so they require special headers.

2+2GT

The "P" in GT40P means the spark plug angle has been revised. While the traditional smallblock head plug protrudes at about a 30° angle, the GT40P spark plug protrudes almost straight. This means the plug would interfere with most exhaust manifolds and headers, which means you must use special GT40P-compatible headers to clear.