So I've actually been doing some thinking and I think I'm going to do three things as it stands right now.
1) The duckslide kit can only support a barrel that has an OD of 0.875", the longbow back has a maximum OD of 0.952". I'm going to have someone remove a bit of metal from the duckslide kit to accommodate the larger OD on the longbow.
2) The duckslide kit only conceals a barrel up to 6" in length. The shortest length tip available for the longbow barrel is 12", I'm going to have someone make a tip to keep the overall length at 6".
3) Finally, I was thinking about buying a roundhead...but instead I think what I'm going to do is have a Blockhead milled down to fit the kit. While this is being done I'm going to see if it's possible to have grooves milled into either side of the body so that I can place bearings in them and allow the duckslide to move on these instead of directly on the metal. Hopefully this will also remove that little bit of wobble.
I think this may be a bit much...but if I can get it done it should be fun...