Quite aside from the wheels, the Touring doesn't have the width back there to remotely handle a 240.
I seem to recall some people have successfully shoehorned in a 200 on the original wheel. I don't see the point, frankly, it would just make the bike handle worse, a 180 is fine in the twisties, you don't have to wrestle the bike down.
Buy a set of Avon Cobras and enjoy great ride quality. Whitewalls if you enjoy the more vintage look it gives the bike (looks great on the Touring, in my opinion.) It's what Triumph mounts on the Thunderbird LE as stock now (exact same wheel dimensions on that.)
The "140 on the front" thing is more personal taste, the bike should steer more willingly with a narrower front. If you like that then it might be worth a shot. I'm sticking to the stock widths, happy with the bike as-is, now that it has better rear shocks especially.