The kit from Doo includes rail extensions and some other junk. The tunnel extension is only about $50. I doubt your aftermarket .080 tunnel for $500 has rail extensions in the kit.
Not trying to steer ya out of installing a new tunnel, but am saying that the two routes are completely different. A new tunnel really means tearing your sled apart completely and rebuilding it. An extension is far less work.
I'm building a 144" out of a 136" frame. And I'm still going with the new tunnel route- ground up rebuild. And it's because I changed my mind about what track I wanted to use, after I bought and installed the tunnel.
I STILL have to rivet on a tunnel extension.
If cooling is an issue, maybe a rebuild is the better way to go- you can install new coolers. Great time to roll the case, if that's what ya want to do. More expensive, that's for sure. A recommendation made to me was to buy new brackets that mount on to the tunnel, if you are gonna go that route- the drop brackets, running board reenforcements, seat/tank brackets, etc. I'd suggest the same- get new ones. It's a lot of extra work to save them- cuz that's what I decided to do, against the advice.
When they install the tunnel at the factory, they use steel, self piercing rivets. I'm sure it's a pretty fancy jig. When they do that, those brackets get "tweaked". You'll end up having to hammer them flat where the rivets get popped through. And then you'll want to repaint them.
The easy route is to slap on an extension. The hard way is to rebuild your sled for that m-10 136". If you're going to install a new tunnel, I'd have to give the same advise about rivets, though, use solid aluminum rivets. That advise, I did take.
Good luck.