I just went through this same thing when I had to replace the original compressor on my '01 last month. The old mounting location for the 2nd gens was on the crossbrace underneath the truck bed. The new compressor that came from PacBrake was larger than the old one, and could not fit in that same spot.
The new compressor came attached to a mounting plate which slides in-between the battery tray and the battery on the passenger's side. So, it's essentially held in place by the weight of the battery. One of the battery tie-down bolts passes through a pre-drilled hole in the mounting plate. I thought it was a flaky arrangement, but it seems to be working just fine.
Hope that helps.