I cut out a rectangular piece (the breech) in a 12" pipe of a 9/16 brass. This was destined to become the barrel (extension) and a sleeve over the 17/32 stub. I had to sand down a little of the casing to accommodate the brass barrel.

Finding a way to keep the existing trigger was a bit of an issue - I'm not crazy about using a key-ring for a trigger. Plus given the location of the tank, a key-ring would be difficult to pull with the "trigger hand". I tried a few things (which resulted in a few unsightly holes in the casing) but eventually settled upon a "wheel" as a point of leverage for my trigger system. This is actually just 2 old rollerblade bearings sandwiching a rubber o-ring and held together by duct-tape and hotglue - I hope it holds. The wheel fits over an existing screw mount in the casing (which I trimmed and wrapped with a tiny bit of e-tape). I ran a piece of (picture frame-hanging) wire from the trigger to the pressure-release pin on the the tank.

I used one (ball-shaped) tip of the "bow" to make a handle for the bolt. I secured this to 2 adjustable ring/hose clamps with some spare screws. These already had holes to accommodate the screws. I put a small piece of craft foam between the clamps and the barrel to secure them to the barrel without warping the barrel from overtightening (because the screw heads are in contact with the outer diameter of the barrel).

I kept the ammo-holders on the side of the gun (had to cut out some plastic to allow it to be screwed back in its original postition. I also kept the scope (but moved it to the outside of the casing and secured it with velcro - so its removable). Yeah, like all scopes, not very useful but it looks cool (and was part of the original look).

Finished! Just had to lube up the barrel assembly.

How it works (in case you haven't already inferred...): using the bolt, the long barrel is drawn forward to open the breech. Dart goes in. Pull back the bolt to seal the barrel. A few pumps and ...fire. In the future, I may also try to add a SSPB in the casing below the barrel.
Shortcomings:
-pump handle - In retrospect, the pump handle being the stock/grip makes for a less than comfortable grip - and the grip was tiny to begin with. I considered installing it underneath the barrel (like the original 2k) but given the shape of the casing and the length of the pump hose, I stuck with the stock. Still alright for two-handed use).
- trigger - the trigger is a little hard to pull (depending on how pressurized the tank is) but I haven't thought of a better, smoother way. Yet. Given that, I might add some rubber tubing to the front edge of the trigger to make it a bit more comfortabel when pulling the trigger.
- range - I'm a little disappointed because I heard so many good things about singled at2ks. This one only gets about 50 feet (stefans). The airseals all seem fine. The barrel might be a little long (about 10 'effective' inches). I know my stefans suck - sometimes they don't leave the barrel, partly because the FBR is crooked and partly because they are not a great fit. I've tried using stefans made from broken stock darts and they seem to do much better. I'll have to tinker a bit more.
Anyway, still not my favourite in my arsenal but I'm happy that the gun is alive and I managed to overcome some technical difficulties. Only my second integration and my first breech so it was a learning experience. Any comments/advice is appreciated.