Benny2.0 - Flat sides to a boiler are not a good idea unless they have "stays". Stays are metal rods the extend from one flat side to the opposite side. The rods are nutted or soldered (or both) and provide support against the outward pushing steam pressure. Most boilers are cylindrical because you then only need stay(s) to support the flat end pieces, saving a lot of building work. Since you can get to all sides of your boiler, you could still add stays. Without them, your bowing will probably get worse (until the metal work-hardens, and becomes stiffer but more brittle). Brittle metal increases the likelihood of boiler failure. Look to a model steam boiler book to get dimensions for stay spacing, rod diameter, etc.

Insulation - Boiler insulation makes a big difference in steam generation. Small boilers, which typically have inefficient firing simply due to the size (flame does not scale down well), benefit from adding insulation. I have used felt, wood strips, and ceramic fiber. All work, and all help. My Krick Patricia operation went from so-so to very fine once I added insulation to it's boiler. McMaster Carr has ceramic fiber, as do model RR live steam suppliers. I used 1/8" thickness "extreme temperature sheeting" Part # 93285k18 on my Patricia. I also wrapped the insulation with green felt, then covered that felt with wood stays, similar to photos of the other beautiful boats in this thread.

search under "ceramic insulation"
http://www.mcmaster.com/

=============
The Graham TVR1A engine is very nice. I, too, assembled one in day (lunch to midnight). I plan to use mine in a Dumas Myrtle Corey sternwheeler. For boiler, I am planning on using one from an Accucraft Ruby loco kit. It's a little small, but I am familiar with its characteristics from my model RR operations.