Another thought for you. If you look at the SBA website (I imagine that you will have joined up by now!), and go to the Steamboat Register and do an Advanced search with the ST 5A as the search parameter. As always you may need to play about with the fine detail of the parameter, but it will eventually give you a listing of all the boats powered by that engine. You can then get a feel for what folk have done before not to mention a feel for the style of boilers used.
Of course the alternative is to put in the hull size and see what comes back.
Hope it helps
That's jolly good advice. Senta certainly performs nicely on Windermere with a 5A in a Flying Dutchman (racing sailing dinghy) hull, burning wood in a VFT boiler.
The other way to look at a 5A is the amount of power available is about the same as two normal people rowing moderately hard (or maybe one olympic oarsman), and you might be able to guess the perfomance adequately from that.