RE: Engine suggestion for dumas vee .40?
no. For flex tube you use either just a brass tube, or a brass tube with a teflon liner. I prefer just a regular brass tube and pleanty of grease myself.
As for motor.... that would depend on what you ultimately want the boat to do/what you wanna do with it. Myself, id put a .60-.67 in it, or at the least a "hot" .45... Its quite possible with a strong .60+ motor it could be a handfull to keep upright, but ya never know i supose. My 36" cat hull with a .65 can be a handfull but its just a sport boat and a bit fun to keep tame and still run it 50+mph.