Looks pretty good to me, should work, only way to know for sure is trial by fire
Drop a bulkhead in there and fire it up :p
The manufacturer for Weldon says the extra bead you used at the seams will not add any strength to the welds... By them, you wasted a bit, however I do the same thing on almost all of my acrylic work just because.
Good choice using 1/4", most of my early models I cheated and used 1/8" for the bends and 1/4 for the sides to add strength... while it worked, after 18 months or so, the 1/8" became brittle, coupled with my clumbsyness I broke a few.
If you use black acrylic for the top of the U you'll find that algae will not be as big an issue with plugging up the works. When designed so tightly they become a real pain to clean.
The front and back chambers could be a little deeper to hold a little more water, but should be okay for 300-400 gph.
BTW test it in the sink or something first... fill both sides and the U, draw the air out with an airline and let it sit overnight and see if any air accumlilates in the U. If you find that you have an air leak, don't go back with the weldon.... it'll hose up your first welds... instead work some 90 second epoxy into the area where the leak is.
Overall... Looks great! Not to mention you get 4-5 shots at getting it just the say you want for the same price places charge for a 'commercial' U-tube and box.