Here's why it's difficult:
* Different engine mounts: The 242 and 258 have different engine block dimensions and mounting points. You'll need custom motor mounts fabricated.
* Transmission: While both engines *could* potentially use the same transmission (depending on the specific transmission in your AMC Eagle), the bell housings are likely different. You'll need to address adapter plates or a transmission swap.
* Exhaust system: The exhaust manifolds and piping will be entirely different and need to be custom-fabricated.
* Wiring harness: The wiring harnesses are different and will require significant rewiring or a custom harness.
* Computer/ECU: The 242 likely uses a different ECU than the 258. You'll need to sort out compatibility and potentially use a standalone engine management system.
* Cooling system: The cooling systems might not be directly compatible; hoses and radiator may need adjustments.
* Other potential issues: The size and location of various components (alternator, power steering pump, etc.) might differ, potentially requiring modifications to the accessories or even fabrication of custom brackets.
In short: While mechanically possible, this engine swap requires significant fabrication, mechanical expertise, and likely a considerable budget. It's not a project for beginners. You'll need welding, machining, and electrical skills, or access to someone with those skills. The time investment would be substantial. You'd be better off searching for a used 258 if you want to keep that engine family.
