Sorry about the slow reply, the spring semester ended and I went to my Dad's shop to work on my '50. Summer semester just started, so I am back.
From what I remember there are four bell housings that will bolt to your block. Keep in mind that there are three series of Ford flathead six motors, but they all had the same bolt pattern on the back of the block. The three series are G 1941-late 1947, H late 1947-1951, and M 1948-1951 busses and 1950 - 1953 two ton trucks. The bells I have seen are:
G car ('41-'47)
G and H truck ('42-'52)
H car ('49-'51)
M New Process 5 speed ('53 only)
There may be others.
The G series passenger cars used a six cylinder specific tranny case and bell housing.
The '48 to '51 six cylinder trucks used the same trannies as the V8 trucks. This bell will allow you to use the T-5 or C4 / AOD conversion kits. I think that the '42-'47 trucks also used this bell. I suspect that the 3/4 ton and larger '41 trucks used this bell. I don't know what the 1/2 ton '41 pickups used for a bell as they were still torque tube drive and should have engine stay rods.
The '49-'51 car bell was stamped steel with the clutch arm in it like a modern bell. You may be able to bolt a late model four or five speed behind one of these. Check the hot rod tech people. Anything that will bolt to a '49-'53 flathead V8 car bell will bolt to this bell.
In '53 Ford started offering five speed New Process truck trannies in F-600 trucks. Since the M was offered through '53 there is a bell that fits. It is a massive cast iron bell that includes the rear motor mounts. My '53 M engine has one of these bolted to it. I doubt you will ever find one of these or want to do anything with it if you did.
I don't know what was used on busses or industrial engines. You are not likely to find a bell for either application anyway.
A thought on your rear axle. '42-'47 Ford pickups had the old style axle with open driveline and parallel rear leaf springs. If you get one of these axles you don't have to do any converting. The only thing to note is that they are a little wider than the '36 axle.
Good luck, Fred.
