You could put the mounts on your motor and measure the distance between the top of the threaded portion below the mount, then measure the distance between the bracket holes in the car frame.

If they are not the same, the brackets are incorrect.

Try a search, this issue was discussed recently (perhaps as much as a couple of years back) with pictures of how the mounts did not line up.

I remember this post because I chimed in when I saw that the pictures were not as good a fit as the mounts in my '64 544 with a B20.

I was also told that later B20 motor mounts are chunkier and stronger than earlier ones - my car presently has a broken right mount - a common issue I was told.

You will need a pair of motor mount brackets from a B16.

Actually, if that 1957 originally had a B14, I'm not so sure. If the B14 and B16 blocks were dimensionally the same, the same motor mount brackets could be used. Eric will know.

I did this in a 1958 that had a B16 - the B16 brackets bolt to a B18 (orB20) block and then work on the earlier cross member.