* Transmission: The automatic transmission from the '99 won't mate to the '94's manual transmission bellhousing. You'd need the entire automatic transmission assembly, including the torque converter, and potentially a new transmission crossmember to support it.
* Computer/Wiring: The 1999 engine is likely OBD-II, while the 1994 is likely OBD-I. The engine computer and all associated wiring harnesses will be completely different and incompatible. You'd need extensive rewiring and potentially a standalone engine management system.
* Other Components: Many other components might be different, requiring adaptation or replacement. This could include things like the exhaust system, driveshaft, and possibly even the radiator.
In short, while some parts might be interchangeable, the project would be far more complex than a simple engine swap. It would essentially require building a completely new drivetrain, a very extensive and potentially expensive undertaking. It would likely be more practical and cost-effective to find a suitable donor vehicle with an automatic transmission.
