Yessir, we disagree somewhat.
The broaching process of the wedge slot in the barrel and arbor distort the material in the same manner which is why they will allow assembly when correctly oriented. It may be somewhat difficult in some cases so reliving the edges of the slot in the arbor will allow easy assembly but it still won't allow 90° rotation (which is not needed for function). Making the diameter of the arbor smaller just for a test (to prove you're right ?!) still makes no sense. We're trying to "tighten" tolerances not add more clearancing!!
As far as the spacer size is concerned, it's not a "one size fits all" item. I've never "measured" to find the size for a spacer, it's a custom fit for each revolver. It's a five minute process to grind an appropriate spacer ( just part of the process). Once fitted and installed, it's done . . . and there'll be a circular witness mark on the spacer where the arbor contact is made.
If you really want to get into the weeds, all cartridge conversion revolvers I do have the arbor removed, the hand relief cut square (instead of the "notion" from the factory), shaft material upset and the arbor reinstalled ( heavily torqued). The barrel assembly installed with wedge in tight before the staking pin is driven in. Once the pin is in, the barrel assy will all but fall off when the wedge is pulled (still can't swing it 90°, don't need to).
That setup will allow a most excellent platform for a Kirst cylinder conversion as well as a "convertible" ( cap gun /cart.).
No disrespect, just different strokes . . . some folks shoot light, some shoot HEAVY ( build accordingly).
Mike