If you intend to use a manual after changing over to an open driveline, then yes, get the tranny, bellhousing, etc... You can't use the stock 3spd with an open driveline anyway.
Yes, as has been discussed a thousand times, the 55-64 car rearends can be used in the 49-54 cars. The '57 is the best choice, the 55-56 ones use different axle bearings, so I would try to avoid them if possible. The 58-64 ones will need to have the original brackets cut off and new spring perches welded on.
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