Bruce,
The problem with using brass on a boiler, especially if it is run with a gas burner is that the brass de-zincs (running on gas accelerates it) and starts to break down and corrode, usually on the inside where it can't be seen. It also hardens and goes more brittle with the constant flexing that the pressure causes.
The reason for the phosphor bronze boiler fittings is for the same reason, it is less susceptible to corrosion and a lot stronger, so preventing the fittings from 'blowing out'.
Also, seeing as to how you are going to make the boiler from a sheet, how are the ends going to be joined together. There are two accepted methods for this, one is to butt joint the ends, then fit a reinforcing strap on the inside that is rivetted either side of the joint, then silver soldered over the whole area including rivets. The other, and more difficult method is to dovetail the joining ends so they fit precisely together and 'lock' the joint, again this would most probably be fitted with a backing strip and fully silver soldered
Here in the UK a brass boiler wouldn't even be thought about, let alone considered, unless it was one of the 'toy' steam engines, that are usually soft soldered and run at very low pressures.
Even at your designed running pressure of 20psi, and a rough guesstimate of 150 sq. ins. suface area (excluding the boiler tubes), this gives a rough total of 3,000 lbs of surface area pressure, and the total improbability of it ever happening, if a fitting was to blow out, or the boiler split, it could cause a major injury either from flying shrapnel or scalding water.
Also your boiler testing seems a bit excessive. We usually take ours to twice working pressure for the initial commision, plus the safety valve is tested to ensure that it is releasing enough steam to prevent the boiler going above the designed working pressure (in some cases people fit two to be on the safe side, just in case one siezes up). In your testing case, again a guesstimation, 150sq ins surface area times 150psi works out to 22,500lbs.(10 tons approx.) surface pressure, I personally wouldn't like to be in the area when you are carrying out your testing, even though being done hydrostatically.
I am not trying to put you off making your boiler, but please be aware of the facts and figures.
John