Want to put a 2000 HEMI engine?

Forget it. You'd be better off using the 2000 truck and installing the '90 body on it, which, of course, is totally different. To install the engine in the older truck, you'd need the Engine Computer, entire under-hood wiring harness, AC compressor that mounts to the engine, custom AC hose to connect the new compressor to the old condenser, larger radiator, and stiffer front springs for the heavier engine.

Because these are two totally different body styles, expect the clutch linkage to mount differently. Drive shaft and engine mounts will be different. Your old engine uses a throttle body fuel injection system which runs a much lower fuel pressure than the system needed for the newer engine. That means customizing some kind of higher pressure fuel pump and mounting it in the tank.

'96 and newer vehicles use a leak detection system for the fuel system. That means adding that to your truck to prevent the Check Engine light from turning on. You'll also need to add oxygen sensors after the catalytic converters. The Engine Computer needs two converters and four sensors. You'll have to add the purge solenoid for the charcoal canister.

Trucks with heavier engines have larger brakes, front and rear. Look for axle and brake parts from a '90 model in the salvage yard that came with a V-8 engine. Those are just the things I can think of. You're going to run into a lot more problems and obstacles. By the time you're done, you would have spent less money if you had bought the truck you want, and it would ride much smoother than the '90 model.