There's no simple "drop-in" replacement engine. Any engine swap would require considerable modification and custom work, including:
* New bellhousing: You'd need a custom bellhousing or an adapted one to mate the new engine to the TH425. Finding a suitable bellhousing is a significant hurdle.
* Torque converter: The torque converter is specifically matched to both the engine and transmission. A new one would be needed, likely a custom-made one.
* Engine mounts: The new engine's mounting points will almost certainly not align with the existing Toronado engine mounts. Custom mounts would be required.
* Driveshaft: The driveshaft length might need adjustment depending on the new engine's size and location.
* Wiring and electronics: You'll need to sort out the wiring harness for the new engine's computer and sensors to work with the car's existing electrical system.
While theoretically *some* engines *could* be adapted (with substantial effort and expense), it's not a straightforward task. Engines of similar size and weight from GM's lineup around that era (late 70s) would be the *easiest* to consider from a logistical perspective, though still requiring extensive modification. Even then, there's no guarantee of success without considerable mechanical expertise.
Instead of a direct engine swap, consider rebuilding or replacing the original engine as a much more realistic and cost-effective approach. Finding a used, rebuilt, or remanufactured 403 Oldsmobile engine would be the most straightforward path for maintaining the original drivetrain.
