Mark, without seeing the plane or the orientation of the tank I will offer my experience. Your description so far is pretty much spot on. The height of the needle valve in relation to the pickup is relevant. As long as your on the correct side of the tank the outboard which you already are, the height isn't much of an issue unless your going inverted. The Fox though is a bit different from other engines. On profiles, the Fox runs better when the pickup is slightly higher than centerline,  A Fox doesn't experience the sane symptoms when mounted upright or inverted as they do mounted on a profile. Some will suggest 5/16"-3/8". While this is good advice visually to start with, the only way to fully know is to fly it.

 This is all pending on where the pickup is soldered inside the tank. Your square tank will feed with the pickup higher than the bottom outside corner. There could be some fuel left in the tank and it might even suck down every last drop. If you plan on going inverted, it could go richer making it difficult to recover to level, it might go leaner. Personally I would just try and fly it with the pickup slight raised off the bottom.  If possible, I would slightly cant the rear of your tank outboard ( Not excessively ) just slightly and make certain it's firmly secured. Keep the tank as close to the rear of the engine insuring you have as short as possible fuel line. Nothing works better than self experimentation. This is your best learning curve. While I've repaired hundreds of tanks,  I can say first hand that I've seen some of the craziest setups defy everything I've known and it worked just fine. Removing a tank and reworking it's feed line is a very simple task which can be done in a short time.

Ken, I chose a 4.5cm width for the tank because the actual room inside the fuse is 5cm wide.  I figured at 4.5cm I can kick out the rear of the tank about a half cm, or about 1/8 inch.  There really is no room for vertical movement unless I do major rework to the fuse.  I don't really want to put that much work into a $10.00 plane, but we'll see.  Inverted is what I'm working towards now.  I'm really just a beginner flyer.  I build 10 times more than I fly.  Which is good if I'm going to be taking the back of the tank off repeatedly.

Tim, I'm sorta going uniflow.  The way the previous owner had the plane set up, he has one hole in the firewall for the pickup, and on the upper inboard side just aft of the firewall he has a cutout for the two fill/vent tubes from the odd little tank he was going to use for his ignition engine.  I'll have the pickup tube matched to the hole in the firewall, and two tubes at the cutout behind the firewall.  One will be a fill overflow tube that gets capped, and one will be a tube that faces forward on one end, and crosses over to the outboard side where I planned to solder the other end of it about an inch ahead of the end of the pickup tube.

I wasn't planning on the .25LA-S on this one.  I'm going to put in a well-used Fox 35 I got at the club banquet last winter, since the engine bearers are already drilled out for a Fox 35.  As I said, I'm just a beginner, so I don't need a full pattern's worth of fuel.  About 3 1/2 to 4 minutes is all I'm looking for.  I hope to actually get off my butt and compete in Beginner's Pattern this year, but OTS or the full AMA Pattern are still a ways out.

Thanks for all the advice, Guys.

Mark

             I chose to write what I did based on Mark's experience. He did mention that he's a beginner and typically beginner's are not flying the more difficult maneuvers. I didn't even want to mention uniflow because of this. While uniflow tanks regulate the head pressure to a enable a more stable from start to finish run, I feel it's better for a beginner to work up to using a tank of this design. There's nothing difficult about using a standard vent tank. This style of tank works well with muffler pressure. The only issue I have is the initial maneuvers on a full tank where the engine is running rich. Many times, a Fox .35 is not very happy on a uniflow tank if the plane is vibrating. Aside from a few I run chicken hoppers on, they're all set up with standard vent tanks. If the Fox is run rich, it's going to be quite thirsty. If you manage to work your way up to the OT pattern, 3 oz. is cutting it close with the Fox. However this will get you enough air time to play with. Just keep in mind that your going to need to launch rich and progressively the engine will lean as the fuel load diminishes.

         Mark, you mentioned that the tank will not move up due to fuselage constraints. If needed, you could shim the engine up from the bearers. This really shouldn't be necessary though because by the description your offering, the Fox centerline should already be higher. This is where it will run fine upright until you go inverted which now places the engine centerline lower than the tank centerline making the engine go rich.  Mark, are you the same fellow responsible for the wonderful builds on the Cox forum? Ken
               

Tim, I'm good at the crashing part already.  Maybe I should just jump up to Intermediate now.  After all, if I gotta crash, might as well crash in a better class.

It depends on where you want to go with all of this.  I was pushing to get good fast, which means always advancing "early" -- you get fewer trophies that way, but it focuses the mind well.

Beginner is a really good place to learn about engine management, and how to conduct yourself at a contest (like, how not to trip over the lines of Experts).  They flying isn't exactly secondary, but it's not the whole.  Once you're past tripping over your competitions lines regularly, you can fly inverted, you can get the engine started in less than a minute nine times out of ten, and you can remember (and do) all of the tricks at least some of the time -- yes, move up to Intermediate.

And by "doing the tricks" I mean that if a judge, who knows what's coming up, can look at your maneuver and say "yes, I guess that's the right manuver", then you've done the trick.