If someone could give a brief outline of how/when the counties change meaningfully enough, we can think of some solutions.
Well, the various creations of unitary authorities and the changes to ceremonial counties in the 1990s (including abolition of Avon, Cleveland and Humberside and restoration of Rutland and Herefordshire) aren't, for the most part, that important for this purpose as the last review mostly grouped unitary authorities with their parent county, continued to use Cleveland and the Humber area as groups and still treated Rutland with Leicestershire. So I see why you used the 1974 map even though it has a couple of features I dislike...
The 1974 changes were implemented in the review which came into force in 1983.
There were a few changes in the 1960s, most obviously the creation of Greater London and the mergers of Huntingdonshire with the Soke of Peterborough and mini-Cambridgeshire with the Isle of Ely. (The two resulting counties were then merged into modern Cambridgeshire in 1974, with Peterborough regaining independence as a unitary in the 1990s, though the boundaries aren't the same as the old Soke.) There were also quite a few expansions and changes to county boroughs, some of which affected county boundaries. These affected the constituency map from the review which came in for February 1974.
Several historic counties were split pre-1974 (or pre-1965): Yorkshire into the Ridings, Lincolnshire into the Parts, Suffolk and Sussex both into East and West, Northants into Northants "proper" and the Soke of Peterborough, Cambridgeshire into Cambridgeshire "proper" and the Isle of Ely. But not all small pre-1974 counties were treated separately for Parliamentary purposes: the Soke of Peterborough was grouped with (the rest of) Northamptonshire, and Rutland was grouped with the Parts of Kesteven.
(That wasn't a "brief outline". Sorry!)
