I got one of the first 3.54 gear sets of the production run by Mike Lempert in 2002. That is, I received and installed the gear set in 2002 but it was two years from deposit to receipt of the product. My continuing thanks to Mike for starting and continuing the project to remanufacture the gears through to the finish. I agree that for anyone who puts miles on their Healey, the modification from 3.91 to 3.54 gears is about the best modification that can be done due to easy cruising and slightly better fuel economy. It has been that for me for 65,000 miles.
Overdrive remained an "option" for Healeys to the end of BJ8 production, so it isn't true that all later cars had overdrive. However, very few Healeys came to North America without overdrive and the 3.91 rear end. Non-overdrive equipped cars were much more common for RHD cars for the UK home market. There were some few "Personal Export Cars" specially ordered by the first owner without overdrive (probably to save a few bucks), and some of these came to North America. Thus, original 3.54 rear ends have always been pretty rare hear. There were other Austin cars that used the 3.54 rear ends besides Healeys, but I think they also were relatively rare in America.