John, I also have a 32nd scale type IX in the works,I don't know the requirements for your engle system.I'm working with 4" poly carb tubing. 6" is going to be a tight fit. I have to spread my hull apart to fit a 6" tube.The I.D. width at the deck at it's widest point is only 5.750" You have plenty of length to work with.If you can go with a smaller dia. you'll leave yourself room for bulkheads/cradles for your cylinder to ride in, also something to think about 4" will fit below the water line therefore only effecting your submerged trim. Any unused space in your cylinder will add to the buoyancy of your boat and have to be counteracted with weight also.Have you figured out where your center of buoyancy is yet? That will determine the center of your ballast tank. just curious.