Motor swap from '97 Ram to '02

I have a 2002 Ram 1500 4WD that had a blown motor, I swapped it with a 360 motor from a 1997 Ram 1500 4WD. Everything bolted up fine, but noe the computer is getting some crazy readings. When the engine fires, the motor will run fine at first but idle around 1100 RPM. After the engine is running it will ping and backfire like the timing is off. The motor was good when I pulled it. The scan tool says that the truck thinks it is the mountains as far as air pressure. The fuel readings was going from rich to lean back and forth. I have repalced the map sensor put in a new tank of gas. Any suggestions would be helpful... Does this motor even match the computer for the 2002?  

I will assume you swapped a 5.9 into a 5.9 right? the pcm used is the JTEC or jeep truck engine controller, was this a complete swap or long-block swap, you should have transfered over your existing hard parts,(throttle body,intake,exhaust manifolds,ETC.)
Scan tool readings for upstream 02 should be switching rich center lean, that is normal,downstream 02 should not be moving even close to that,(this tells the pcm if the cat. conv. is functioning properly).
You did not state if the check engine lamp was on,or if there is any codes in the system?
As for the ping and backfire, first thing did you use the trucks original flexplate from the 2002,(I hope). if so I would start looking at the fuel pressure first, assuming no codes and proper flexplate, with the scan tool you can access what is called adaptive memory (commonly called fuel trim data) check the cells and look for double digits in the positive range, example +12 0r +15, there are 14 total cells to view, + means the pcm is trying to add fuel via wider injector pulsewidth, - digits (-12) means the pcm is trying to reduce fuel.
These double digit plus numbers are a good indication that you should be doing a fuel pressure test. also do a fuel volume test, lots of times the pressure is there but the volume is not. we have replaced a ton of fuel pumps over the years,
Next question, what happened to the old motor, Burned piston??  

One other quick note, these intake manifolds 97-02 were known also for blowing out the bedplate gasket causing internal vacuum leaks leting the motor ingest its own oil, check idle vacuum reading with a mechanical vacuum guage (19-21 in hg) is normal with the motor shut off open the throttle and look down the throttle body @ the floor of the intake if you see oil or heavy wetness the gasket is done.