My measurements are ....55 1/4 at the top bow socket at the top of the tub, 56 1/2 give or take a little at the fore-aft support rod on the rear bow.....about the same at the front bow where the fore-aft rods connect
I also have a Walck's repro top frame and a Beachwood enclosure....
These things are not perfectly square...they have a bit of a covered wagon look from the rear...
What I did was to assemble the rear bow with the cross piece, insert one side partially into the socket, then just flexed the assembly until the other side fit into the socket...I think I used a little wax to help the tube slide into the socket....
It sounds as though one of your sockets may not be set vertical...