I agree with Adil on the OS engine for the Dynamax although I have seen a couple of planes powered by the K&B .82 in the Dynamax that did well. Adil is saying to remove every other stator, not rotor blade. I have heard of this modification but I've never tried it myself. It does look like it would flow more air.
Are you scratch building an airframe or are you installing this in a kit?
I have made several bifurcated thrust tubes for Dynamax powered Miller F-4s and you want about 2 5/8" outlets. I built one that was only 2 1/4" outlets and the static thrust was reduced dramatically. On a single outlet tailpipe you can go a s small as 3 3/8". On my Regal Eagle bifurcated thrust tube, the 2 oulets are different sizes. I used an angled header to run the tuned pipe out the left outlet. I made that outlet larger to allow for the size of the tuned pipe. It works very well.