My DL-50 worked great with a 22x8 (6900-7100 rpm) and OK with a 23x8 prop (~6300 rpm), but would lug down with much more prop than that and overheat, and carburetion was less "accurate".

I REALLY doubt it will turn any 26" prop anywhere near 6000 rpm, but anything is possible. The 3W 55 is quite a bit stronger than a DLE-55 (flame suit on...), especially in its ability to turn a larger prop, so that engine may get away with it.

If the DL can swing a 24x8 at 6000, then you are correct it should be able to swing a 26x6 around ~6000. If you use the DLE in this way definitely get it enough cooling air flow (particularly keep the crank case cool to preserve the bearings) and don't skimp on the oil. Perhaps back off the timing a bit. Pick a prop with a very light load, such as a Dynathrust, Zinger, MA Scimitar (which would look somewhat scale), etc.

One more thing, is the thrust speed going to be fast enough to keep the plane in the air? I typically try for a thrust speed that is about 2.5 - 3 times faster than the stall speed. The thrust speed shows about 34 mph, which will certainly increase a bit in the air as the prop unloads. If the stall speed is expected to be in the low teens, then this should be about perfect for your plane. However if the stall speed is in the low 20's, there might not be enough excess "power" to get the plane out of a sticky situation. That said, the DH-4 will be so adept at "flying on its wings" that this may not be a problem in any way.

p.s. I have always enjoyed your articles in RC Report, they were always fair and honest and seemed less like an "advertisement" as many other reviews and articles can be.