The Mercury cars were generaly always a step up in the strength department. They had the 255 CID engines in the flathead era and the 256 CID at the beginning of the Y-block era. The 292 came into use in 1955 with a 4V carb set up and FoMoCo was likely planning the stronger case transmission before the engine CID changes. When the 312 came along Ford was ready for it. Heavy duty engine packages called out for heavier duty transmissions. In 1957, the 368 came on line for Mercury. The Lincoln 341 and 368 applications started the trend in 1955 along with the higher performance T-bird & likely some fancier station wagons. As was previously mentioned, the Big Lincoln used the Turbo Drive which was Fords way of dealing with the GM 4-speed Hydromatic plant fire that ended production for the Lincoln cars. The large case followed a year or so later to beef the Lincoln up for later and larger powerplants.