Ah yes, "the correct bellhousing"! Have you found whether they exist? I remember '51 Ford ads listing a Ford-O-Matic for the flat 6, but were any ever really built? As far as using a standard trans bellhousing, I think the critical dimension would be the depth (from trans face to block face). But with a mill, lathe, welder and patience, 'most anything can be done.
I am 99 percent sure some were made and put up for sale because I have official Ford sales literature for 51 Fords,and the sales literature brags about how "If you buy a Ford you don't have to pay extra to buy a V-8 to enjoy having an automatic transmission in your car,because it is an option with 6 cylinder engines in 1951 Fords!"
Almost certainly not a direct quote because I am writing from memory,but that is the essence of it. Henry Ford was known to be cheap/thrifty,and I have a hard time believing he approved of spending the money on advertising to sell cars he wasn't building.
IMHO,it just wasn't a popular option back then because people so cheap they bought a flat 6 Ford instead of "having V-8 POWER!" at just a few bucks more were unlikely to order anything other than a standard shift with overdrive.
I am GUESSING that Ford blewthem out at bargain prices to fleet buyers to get rid of them after the initial production,and then just stopped taking orders. After all,the following year they would be offering a OHV inline 6 engine with more power to replace the flathead,and two years later would be dumping the flat V-8,so it may have just made business sense to quiety drop it than spend big bucks advertising and trying to sell "a horse that is already dying".
As for using the truck bellhousing and starter plate,I'm not even going to try to guess. I bought them off ebay because it SEEMED like they might be the easiest way to create what I need,and I will find out once I have the parts car engine sitting blocked up on my shop floor so I can see what there is to be seen and measured.
Since I may be losing my left leg being able to drive my 51 coupe will require an AT. It is either do this or put a V-8 in the car,and I want to keep the flat 6.
Then again,I do have a 339 cube Lincoln flat V-8 sitting out in the yard,and GM hydro trans from the early 50's will bolt to it...........