Here's what to consider:
* Engine Mounts: The engine mounts will likely be different. You'll need to either find engine mounts specifically designed for the new engine in your chassis or fabricate custom mounts. This is often the most significant hurdle.
* Transmission: The transmission (automatic or manual) from your Pontiac engine may or may not be compatible with the replacement GM engine. You might need an adapter plate, a new transmission, or a complete drivetrain swap.
* Bellhousing: The bellhousing that connects the engine to the transmission will almost certainly need to be changed.
* Exhaust System: The exhaust system will likely need modification or replacement to fit the new engine.
* Wiring Harness: The wiring harness may need modification or replacement to accommodate the different engine's sensors and electrical components.
* Cooling System: You may need to adapt or replace components of your cooling system.
* Power Steering & Accessories: The placement and type of power steering pump, alternator, and other accessories may require adjustments or replacements.
In short: While GM uses many interchangeable parts, swapping engines requires mechanical expertise and potentially significant fabrication. Researching specific engine swaps for your '66 LeMans (e.g., "LS swap '66 Pontiac LeMans") will yield more specific information and resources, including forums and vendors specializing in these conversions. You'll likely need a shop manual and potentially professional assistance.
