The 410, 420, 540 and 542 all have the same rear bed length of 1.90m to 1.97m or 6ft 3ins to 6ft 5.5. The difference is the curve of the rear corner.
The standard spec. 410, 420 and 540 are 2ft wide as singles but if you opt for the multi-function set up the singles can be 2ft or 2ft 6ins wide.
Most UK vans have the interior sprung option whilst the front dinette is foam only. I've slept on both and they've been fine. It's possible the foam might degrade quicker, though on both our 540's you could feel the interior springs under the cover if you ran your hand over them, but it made no difference to sitting or sleeping comfort.