Allen hit it on the head
The problem lies in the fact that GM changed the shape of the cross member under the bellhouse AFTER the 53 model year,They were sorta flat.
Beginning in 54 the crossmember dropped down,then flat,and then back up sorta looking like the bottom half of a stopsign.
The I6 bells have the same bolt patterns for the block and trans thru 1962 the only change being at the trans mount.
53 and older the mounts bolted to the sideof the bell
54 and newer the mounts bolted IN BETWEEN the bell and crossmember like a pad and a bolt ran up thru a lower half,the crossmember and threaded into the bell
