The '54 and '55 1st frames are different than either the earlier or later frames.
The difference between a '47 through '53 and a '54 and '55 1st is the kickup over the rear axle, and I do not believe anything is different at the front. In addition to the flat side tops, the bed is also deeper, and that is why the kickup is lower. The bellhousing crossmember is different after '53 also, that that does not impact the MII install.
The '55 2nd and later is completely different and would not work under a '54 or '55 1st cab, though the bed would fit nicely...
These details all assume a 1/2-ton truck but are pretty much the same on a 3/4-ton. The 1-ton has straight frame rails all the way back '47 to '55 1st.
