Why not use Sky Shark 5PTs (or for lighter winds, 3PTs) for the outer spars?
They use the same ferrule size as a P-300 or P-400 and the tapered ends should fit in the Deca's caps.
I'd suggest one of the newer PX series spars for the centre:
The new tubes have a new woven look, they are very much the same as the old P series tubes, same ID and OD. The big difference is the tubes are made with a higher modules of carbon which makes them 15 to 20 % stiffer that the old P series tubes.
The New Sky Shark™ PX Series Straight Competition Air Frames.
These tubes can be cut and ferruled anywhere using a solid carbon rod. They can also be ferruled with any of the New Tapered tubes except the 2PT. These tubes work great for building sport kites as well as single line kites.
All of our PX series tubes are non-tapered, with the same inside diameter as our tapered tubes. You can join any of our PX series tubes with the P series, or tapered tubes. These tubes are made in much the same way as our wrapped tapered tube with the exception of materials used . The PX-series also use a different layering of materials which makes it possible to cut the tubes anywhere and ferrule them without breaking out the wall, unlike the tapered tubes.
All the new PX series tubes come from a new manufacturer and we think the quality is better than ever.