Going back to the Airbus question :
The A340-500/600 have a lot more differences with the A340-200/300 than just a fuselage plug : (and speaking of plugs,
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Photo © William Ronciere
On the above picture, you will notice that the wing is a totally different design , i.e, larger wingspan, taller winglets, different materials (different composites), etc..
The A321 uses the same wing as the A320.
Totally different engines; Rolls-Royce Trent 500 whereas the A340-200/300/8000 had CFM 56-5C2 or 5C4 engines.
The A321 uses either the same engines as the A320 or "upgraded" evolutions of that same engine, i.e CFM56-5B1/B2, etc...
The A340-500/600 cockpit have a slightly different layout due to the use of LCD
screens as compared to CRT screens for A340-200/300's.
(however, the recent A330's and A318's also have LCD
screens, maybe recent A340's also have them).
The FMC's are different, and the MCP
seems different too.
The A321 cockpit is exactly the same as the A320's and A319's.
Hope this helps,
Fly to live, live to fly - Air France/KLM Flying Blue Platinum, BMI Diamond Club Gold, Emirates Skywards