Turn your gusset sideways so that it becomes not only a connection for the TS, slotted as you described, but serves as a web stiffener for the W14.  The gusset does not have to be colinear with the flanges to work properly.  If necessary, though not likely, you can provide a smaller triangular gusset to the gusset plate in the parallel direction (you'll end up with a "cross" shape).

You might have to provide a thicker gusset for this option but will be easy to fabricate and less expensive than the WT sections.

I suppose that I should ask the question.  Is this a truss for a highway bridge or for a building?

If this is a truss for a highway bridge or a very dynamically loaded structure, I would be very leary of welding gusset plates to diagonals and chords.  This is because the welds that are in areas of large tension stress or stress reversal are prone to fatigue, if the welds are not designed and detailed properly.  In this situation, I would choose a different shape for the diagonals (ie 2 channels or angles, or another wide flange) and utilize a bolted gusset plate connection.  Historically, there have been problems with respect to fatigue in slotted tubular connections that are welded.

Dr. John Fisher from Lehigh University has written many articles and a few text books that investigate fatigue and steel connections.

If the truss is for a moderately "static" structure, I feel that Ron's suggestion is a good solution.  There are a few books out there that can assist you in these tubular connection.  One that comes to mind is from the AWS (American Welding Society) and is written by Omer Bloddet, and I think it is called "Design of Tubular Structures"

Another possibility would be to weld a two gusset plates to the flange of your beam. The gusset plates are distanced from one another about the same width of the width of your tube diagonal. You then use bolts long enough to go through gusset plate + tube + gusset plate. This system is easy to fabricate and also allows alot of flexibility during installation. Remember that your bolts will be acting in double shear.

One other thing I believe that you should consider with bolted connections and tubular shapes.  When torquing bolts on tubular sections, the tension in the bolt from torquing utlizing the "turn of the nut" method can actually crush the tube section.  What I have done in the past is to place a steel sleeve (or section of pipe) which has an inside diameter slightly larger than the bolt (1/16" to 1/8") in the tube.  This sleeve is placed in a hole that is slightly larger than the outside diamter of the sleeve, and is groove welded to the outside faces of the tube.  This detail was developed when I was designing a several tubular trusses used for carrying Variable Message Signs over highways.

This will allow you to place gusset plates on either side of the tube and the connection will act in double shear, as civeng has indicated.

This may seem excessive, but I was really suprised to find that this tension in the bolt, even under the turn of the nut method, was enough to crush the walls of the tubes I was using.

If you are ineterested in seeing the bolted details I used, just let me know, and I can also send you a copy of the calculations.

This is one of the programs that can do bearing checks for tube to WF connection:  http://www.lanxun.com/pce/stiff.htm

You're first option is interesting as I recently saw the SAC Museum (US Strategic Air Command) just west of Omaha, Nebraska which was framed with very large arched trusses made up of wide flange chords and wide flange diagonals.  The connections were made with single gusset plates welded to the webs.

I contacted the structural engineer and asked him about the issue of web bending due to the gusset plate connected perpendicular with no stiffener plates.  His contention was that the truss was designed conceptually as a pinned jointed frame and wherever a diagonal connected to the chord, there was another diagonal adjacent to it that took away all of the force perpendicular to the chord.  Thus, the web never experiences a net force that would cause bending, but only a net longitudinal force.

I'm not sure I agree as the "theoretical" frame is one thing, but the actual behavior of the truss is another.  I would suppose, though, that if there was a small net force, the web would distort and the forces would re-distribute.