Hello Daniel, and welcome to the forum !
Regarding measuring up for your new air awning, it’s not just about the length of the horizontal part of the awning rail.
Awning rail height is also a factor, but having a swift group van like yourself and also having 2 Vango Air awnings, I can safely say that this won’t be an issue, but what is an important thing to consider and one that is easily overlooked is the position of the windows. You really don’t want the awning to end where you have a window.
Although your van is in storage, why not arm yourself with a tape measure and visit a local caravan dealer who have other vans with an identical layout to yours on display (it doesn’t have to be an Eccles Sport 640, as there will undoubtedly be other Swift vans marketed under the Sprite, Challenger and Conqueror names with the same dimensions, as you will be able to see with a quick search on Swift’s website) and measure up. Make a note of the dimensions of the air awning you are considering, and make sure that the awning would fit without conflicting with any windows. It’s also a good idea to have a look at, and take measurements directly from display awnings that are the same as the ones you are considering as they also occupy a little extra awning rail space on each side which has some additional webbing in the form of a gusset to keep the whole thing taut. These end sections don’t necessarily have to be confined to the horizontal section of the rail, as ours isn’t at the rear, and this doesn’t present a problem.
There are some forum members who through experience would advise against purchasing an air awning but to the best of my knowledge, the vast majority of these are relating to Kampa awnings, and not Vango. I only have experience of the latter, and touch wood, have never had any issues with either of mine,
EDIIT: After a cursory glance online, I can see that the Sprite Quattro DD, Swift Challenger 640 and Swift Conqueror 640 are the same basic van as yours Daniel and so you shouldn’t have too much trouble in finding one locally to measure up.
I Hope this helps, but if you do need any more info, or advice, don’t hesitate to ask, as you’re certainly in the right place !
Ic.