To put the 96 drivetrain into the 99 truck is not just a simple engine and trans change over
You have to swap the fuel lines and the fuel tank too (99 has one fuel line, 96 has two)
You have to swap the pedal assemblies to go from 99 auto to 96 clutch
You have to swap the entire wiring harness, not just the drivetrain harness but the under hood power wiring as well because the 99 also uses a different power distribution box and components
The 99 uses a different instrument cluster (digital signals) and some of the wires moved locations, so you have to swap those and change the wiring around
That is just off the top of my head without being there, this is a major project, It would be easier to swap the 99 body onto the 96 frame and use the 99 springs to fix the lean
You must be in a larger city up there, good luck with whatever you decide
If you are crazy enough to attempt this then EF would be crazy enough to help, but we also offer sane advice