* Different Bolt Patterns: The transmission's bellhousing, which connects to the engine, almost certainly has a different bolt pattern. Engine and transmission pairings are specific. A 1986 transmission's bellhousing won't likely match a 1994 engine.
* Different Input Shafts: The input shaft of the transmission, which connects to the engine's output shaft, will likely have different dimensions.
* Internal Differences: Even if the bolt patterns and input shafts *were* compatible (a very big if), internal components like gear ratios and overall length might be drastically different, causing problems with the driveshaft and potentially the transfer case (if applicable).
* Electronic Controls (Possibly): 1994 trucks were starting to incorporate more electronic controls for transmissions. The older 1986 transmission might lack the necessary components to interface with the 1994's computer system.
In short: While there's always a *theoretical* possibility of making such a swap with extensive fabrication and custom machining, it's not practical and would be prohibitively expensive. Finding a transmission specifically designed for the 1994 GMC truck is the far simpler and more cost-effective solution.
