Thanks Swarfy,
I'm gradually getting to the bottom of this. I think.
The frame slots for the spindle are in fact 11/16" (my previous typo) and that means that the swinging arm spindle will fit as it's also 11/16". So rigid and swinging arm are the same.
I imagine that the plunger spindle was beefed up to 7/8" to handle the additional bending forces introduced by what is effectively independent suspension, which must only be exaggerated if a sidecar is attached.
The swinging arm spindle, off-side spacer, stub axle and nut should all fit...
I will need to kit out my crinkle hub with swinging arm bearings (and centre spacer?) and the QD hub with the same. I now understand what Rusty Nuts was talking about and I'll play with the off-side spacer, as he suggests, when I get it all assembled.
Will keep you posted as it may be useful for others.
H