The only obvious difference between the 2.5 & 3.0 litre motors were the different carburettors and air filters
Mk IV Pages
2.5 litre V6 Zephyr engine bay
3 litre V6 Zodiac engine bay with spare wheel removed
Artist's impression of 2.5 litre V6
Cutaway 3.0 ltre V6
See also
New Big Fords 
"Autocar" 22 April 1966