* Bellhousing Bolt Pattern: The bellhousing bolt pattern, which connects the engine to the transmission, is different between the 390 (FE series big-block) and the 289 (small-block). This means the 289's bellhousing won't bolt directly to the transmission used with the 390.
* Transmission Compatibility: Even if you could somehow adapt the bellhousing, the transmission itself might not be ideal for the 289. The 390 is a significantly larger and more powerful engine, and the transmission may be sized for that higher torque output. Using the same transmission with the smaller 289 might be okay, but it's not optimal and might lead to premature wear on the transmission.
Therefore, swapping a 390 for a 289 requires significant modifications, including at minimum a new bellhousing and possibly a different transmission or at least an adapter. You'd likely need a bellhousing that fits the 289 and is compatible with your current transmission, but that's not guaranteed to exist.
