According to the photos I see and what I can read in Italian in Abarth 850TC e 1000TC:
Group 5 existed only in 1976. It differed from the Group 2 specifications Abarths had run in since 1963 in allowing many more modifications. From the pictures I see you can't tell by looking because they involved brakes and other things you can't tell in a photo. Photos from 1976 show Group 2 and Group 5 cars that look identical, and both have the stock hoods.
The fiberglass rear wing appears in 1972, but most cars continued to run the stock engine cover elevated. The stock cover is actually larger and I wonder if some people thought it was more effective.
Wide fiberglass fenders were homologated in 1970 and most cars in the photos have those from then on. But "narrow bodied" cars continue to be seen as late as 1973.
1970 seems to be a transitional year regarding the roof paint pattern. That is when the "lines around the edges with 2 lines crossing" pattern appears (I believe first on the factory cars.) But the checkerboard (or no special paint pattern) continues to be seen in 1970.
So I think to make an accurate replica it is more a matter of saying "I want the car to look like a 1000TC of 1970" for example.
My car looks like an 850TC of of 1965.