Wider is better for dredging right? the quicker you can diffuse the flow and slow it down the quicker your heavies can fall out. This is the principle behind dredging. With sluicing or high banking it is different. Narrower sluices with a wide intake increase the flow because the water is already close to the right speed for the gold to drop out but needs to be a bit faster to flush the lighter materials... Correct me if I am wrong here. I am going to be going through a HUGE learning curve when I finish my sluice/banker and take it out there because I have not ran my gas pump yet and I don't know how it is going to be to adjust the flow.
I really like the idea of adding an angle bubble to the box btw. I will be adding that so I can set it up in my garage so that I get the right slope when I set it up in the field.
Your description of the principals involved between dredging and high banking is sound.

There are always exceptions to any rule however.
As far as adding a bubble to the highbanker. Pretty much un-necessary. Its easy to see if it's level side to side just by watching the water level in the sluice if it's the same on both sides or not, or by watching the over flow whether or not it's even across the end. Also keep an eye out for if the material starts collecting on one side or another after the legs settle in.
I sometimes take along 4 bucket lids to set the legs on when I know I'm going to an area with soft ground to prevent them from sinking.
As far as the angle lengthways..........
You usually have to fine tune the angle and the flow in the field while running material. If you are getting too much buildup of material in the sluice then adjustments must be made, same holds true if the material is clearing out too fast. Adjustment really depends on the material being run. Mostly gravel, mostly sand, dirt sand gravel mix, etc, etc. all will require a sleightly different setting for maximum efficiency.
With those 1" riffles of yours you are going to need about 3" of drop per foot of sluice to be in the ballpark, then fine tune as needed.
GG~