I believe there would be no software mods, let alone soldering irons - though physically removing heatsink assemblies may be challenging, some gripes about some screws on the memory cages being tough to remove. People report successful swaps on their MacPro 1,1 without a firmware update where the CPUs were not properly labeled in About This Mac, but there's a thread on
putting 2,1 firmware on 1,1 machines
that should cause them to be properly recognized/reported.
I've done both mods - replacing the dual core CPUs with quad core as well as the MacPro2,1 firmware update. It's fairly painless - just remove the heat sinks, swap CPUs, apply new compound and reinstall. I didn't find the mounting screws overly difficult with the right tools at hand.
@recondas - I have a few (bad) memory modules you could steal heat sinks from if you want them. Was going to toss them anyway.