A's have a square plastic cover in the trunk that allows access to the innards of the rear frame rails, at least my 65 does. You might either fish a bolt thru so you clamp only one side of the rail, or insert a steel tube for a thru bolt support. For fishing, you can wrap a thin steel wire around the bolt threads to feed it thru. Many trailer kits come with that. I didn't use an innard tube on my Newport, but have large square nuts on the outside that spread the clamping force to the edges of the rail.
