I have a 99 dodge 1500 with 204000 miles seems to still run good. but what is expected life of the 5.9 gas motor, does anybody, know? and second is it possible to swap motors with the new 5.7 hemi if you change computers and all?
thanks..for any info you all have.  

I don't really know about the newer ones, but the 80's ones ran forever. Mine had 298,000 when I flipped the truck. I have no doubt it'd run today if I hadn't totaled it.  

With routine maintenance? A very long time.

Yes you can swap in a 5.7, but it will require alot of work. Motor mounts, tranny mounts, tranny, practically all the wiring in the truck, ect... it would be a cool swap though.