Would I be wrong in thinking a MK1 front end is slightly smaller than say a MK3 onwards?
Yes you would be.
There were many changes over the years, but the shell itself didn't change in the 41 years of production. The biggest changes were the Clubman, the Traveller/Countyman, the Pickup and the Rover Cabriolet. One thing to note is that the Mk1 shell had a smaller rear windscreen and external door hinges. Most of the changes to later cars can be modified fairly easily, such as the lip on Mk2 onward bonnets and the larger gap for Mk2 onward rear lights.