5.7 Hemi into an old Suburban.

If you have the complete donor truck my suggestion would be to use as much of the wiring, harnesses, and modules as you can. You do not need to flash the PCM unless your donor truck has security and you are removing that module. One issue with getting the gauges to work could be vehicle speed signal. This is output from the ABS module which needs inputs from the wheel speed sensors. If the track width from the ram and suburban are close I would also the axle to a provide the speed signal. Good luck!