Biggest problem is the heater pipes exit, DIRECTLY behind the Left hand head. With the bulge in the firewall, there is very little (if any) clearance.
one option is to bash in the firewall (and break the heater box), remove the air con evaporator and run the heater pipes through the air flap and out the air con hole.
the engineering side is the easiest, as the body never actually changed, and all componants are bolt in.
the hard part is Pollutions (EPA). the year of things matter as alluded to above.
The law states you must meet the EPA requirments of the date of build of the newest componant.
So being an XF, you have to meet 84 or later emissions....no clevo built that late.
If you dump in a 5.0 roller moter from say an Ed then you must meet the Ed's emissions.
By and large it is pretty easy to comply. For a clevo, STRAIGHT LPG will see you square. For a 5.0 roller, you will need to leave on the injection, and use the cats and evaporative emissions. None of this is overly hard, and doesn't involve rocket science...all simple bolt on stuff.