Ok... i would call it plausible. (Mythbuster style LOL), cos
given that the whole F430 body came as a kit, that would mean,
according to you, 3 months dedicated to re-work the chassis length,
wheelbase and tracks front and rear.... which still somehow sound
impossible. 
You can
convert a 512 BB into a 512 TR or 308 to a 288 GTO... but never
heard of 612 into a 430?
okok...even if that is possible, to move a 5.7L V12 engine to the
rear and reconfigure the whole drivetrain and transmission just
sounds plain... crazy. OR if that car has its engine left in
front.... lots of cooling problems, unless the radiators are
re-postitioned or split half and placed at the vents on the sides.
And i definately do not believe the front end of the 430 can
accomodate the V12... would be a damn tight fit.
If the fact that the 612 was replaced by a 430 ALTOGETHER as
WHOLE... ah then it sounds more sensible. 
