We covered this on another thread recently, but no problem.
By all means, go with the uniflow. You can always run a uniflow standard if you want by not plugging the overflow (don't know why you would want to, but it gives you a chance to compare), you can't switch a non-uniflow without opening it up and replumbing.
Go with all tubes forward. Then make a tank compartment where you can slide the tank in from the front. You wedge the tank in, top and bottom with thin shims and this allows you to move it up or down as needed to even out the runs.
First off, an open univlow tube in the cowl will work. How do engines with the carb or venturi in the back work? Same way. But you can (and probably should) simply put a hard mounted vent on your airplane somewhere and then simply connect it with flexible (fuel) tubing. Most people seem to put this vent through a motor mount and out the side (then bent to face forward). Of late Matt has been putting his in a scoop in the top of the plane mounted just behind the prop. It makes for a convenient place to fill, and keeps the wind from affecting the pressure in the tank.
You will also need to bring the overflow out somewhere. This is not critical except that you want it to overflow to the ground and also to be easily plugged. Some people will bring two tubes out the side for this purpose. One is the inlet tube for the uniflow tube. The other is the overflow. You can attach a tube from a small bottle to the overflow and recover any overflown fuel in this bottle for use again. Just remove it when your tank is filled and put the "cork" on that spot and you are ready to fly. (By the way, Matt brings his overflow tube out the bottom through the cowl. This makes it essentially invisible when viewing the plane on the ground.
Why I don't build an army of Twisters is beyond me, but I'm a sick, sick person 