I'm currently running the pipe a bit long 7-7/8 from the center of the pipe's belly to the center of the glow-plug. I've run it at 7-1/4 on "fast" water and the boat will scream when the needle is right. Keep in mind that these dims are for a Traxxas TRX 2.5 running a cut-down Prather 215 on a CAT hull.
I'm sure that on a very free-running hull like a Rigger you will be able to run the pipe shorter and still pull the prop.
As far as the MIP Stinger Pipe is concerned it's a bit restrictive but has the back-pressure required to load up good at high RPM's so keep the props on the small side. If you drill out the stinger out a bit it may help, also.
Edit:
The header that I'm using is the one made for the Associated Nitro TC3. It's ID is 12mm and the header length is quite short for a 180-deg headers I have not needed to cut it from the "stock" length which is 3-3/16 from tip to the end of the exhaust flange.
