You have a WA postcode as your location so I guess you might live in Warrington, if so all the local canals won't be a problem Bridgewater, L&L, T&M and the Lancaster can all take a boat of that size, however, if you want a "go anywhere" boat then there are locks where even 6 feet 10 is tight.
I'd like to know why this boat has spread and what's been done to stop it keep happening even more, as the other guys have said, check it has been measured correctly, find out why it has spread and if there is a reason and it's been fixed then allow for the fact it will have a limited appeal on the wider network and hence a lower value, I have no idea what an insurance company might say but generally if you don't tick all the boxes as "normal" insurance can be an issue and they might apply special terms.
You can PM the details of the boat if you don't want to put them on here to anyone who's opinion you might seek, but generally the odds of someone on here trying to steal your "spread" boat are so remote I'd go public, post them and get so input from those in the know. All boats have a history there might be a previous owner on here with something to tell or someone who's had a survey done on it already and is still running away, your call but I hear the distant sounds of alarm bells on the width thing.
K