Hi,
When I was drawing HO scale plans for models of the C16 and the class 56, the most noticable difference was that the boiler on the Class 56 is 49" in diameter and the Class 60 (C16) has a 50" boiler.
This works out to about 0.30mm in HO scale.
Differences in domes and other things are easy to change.
Doug vV