Will a 1966 Pontiac 389 block bolt into 1978 Trans Am?

No, a 1966 Pontiac 389 block will not directly bolt into a 1978 Trans Am. While both are Pontiacs, the engine mounts, bellhousing bolt pattern, and potentially even the overall dimensions changed significantly between those years. The 1978 Trans Am likely used a smaller Pontiac V8 (like a 400) with a different block design and mounting configuration. You'd need significant modifications, including custom motor mounts and potentially a different transmission and bellhousing, to make it fit. It's a substantial undertaking, far beyond a simple swap.