I apologise for this response in advance - but the best and easiest swinging arm frame to fit a Square Four engine into is of course the Healey 1000/4 frame.
I sold my swinging arm Square Four kit last week. The argument went "Why do I need a swinging arm Square Four when I have a Healey?" (The swinging arm Square Four kit involved a 1954 swinging arm frame modified to take the Square Four engine).
36 4G, 37 VH, 53 ex ISDT KHA (project), 54 KH(A), Healey 1000/4 (project)
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.