Yep, Da Dirt's last season or two, he used the .25FP with a header and aluminum tuned pipe from MACS products. I have the pipe data filed someplace, and I believe he used an APC 10-3 prop. He flew this powertrain on scaled down Impacts in the 450-500 square inch range, called "Wimpact 450" and "Wimpact 500", oddly enough. You should be able to do a search for those names and find all the data. If that doesn't work, I would expect DD would be willing and able to give you specifics...if you're willing to follow the instructions exactly.
If you're apprehensive of using a tuned pipe...don't be. It's not that difficult to setup, and once that's done, it is no different than any muffler. Another potential setup would be a MACS Muffler, which has been used with great results on .40FP, Tower .40, and .46LA.