I don't believe there was ever a 34X from the factory. If it is a 78, it is the IOR 34, which was a 3/4 Tonner (meaning the IOR rating was about 24.5), and the flush deck version was fairly competitive for the first couple of years. The IOR rule was changing very quickly those days, so most boats had very short competitive lives-at least at high level competitiion.
These boats were best upwind, fair dead downwind(as long as it was not too windy), and worst at reaching. This is due to the extreme pinch at the stern, which makes for a short waterline (for the length) and creates a lot of turbulence/drag coming out from the stern area. Again, this was typical of the genre-if you had a cleaner back end, the boat would be faster, but the IOR rule would pick it up and the rating would go up. That was the game we played back then with the ratings: how to make the boat "look" slow when measured, and still have the best speed obtainable with those hull shapes.
So, the flush deck was fastest, the 34T (trunk cabin) was still fairly good at the club level.
Hope this helps,
Seth
