the loom that connects to the pcm is the "engine bay loom" complete with the fuse/relay holder, this connects to dash loom by the big multiplug below the PCM, then the only other loom is the engine itself.
if you use the 155pcm though, you may need to alter dash loom to accept the mondeo clocks, if the transit ones dont work.
ARE your clocks the old style ones
if you use your 125pcm with seperate injector module(have you got a seperate DELPHI module somewhere

) then i think that putting the 155ps's mapping into the pcm like we said before would defo not work.
if it were the same type of pcm though like the st one then you may have a better chance.
when they introduced the 125ps tdci, the first ones were visteon pcm's like a TDDi but with seperate injector module, they changed this after only 9months to the full delphi PCM, they did the same with mondeos.