First of all, everything you said sounds fine. Adrian's house is light weight and would have the affect of moving the CG slightly aft ...but not significantly. Most of the weight in the the house is near or just forward of the CG, which means moving it around (or lightening it up) doesn't have much affect ...the lever arm from the CG just isn't that large for this stuff. That said, if you are thinking of having deep fish wells aft, carrying several hundred pounds of ice, then it sounds like the aft belly tank (as shown in the drawings) won't fit back there ...your fuel is likely to end up in the single forward belly tank or possibly in box tanks in the bottom half of the settees that you mention, and/or in saddle tanks located under the sheer deck on either side. Either way, having the fish boxes in the cockpit area and fuel located amidships as mentioned should work fine. I would not however suggest moving heavy items such as fuel towards the bow, but they are fine in the aft 2/3rds of the pilot house area. The boat's large enough to 'not notice' this stuff too much, but you don't want a boat to trim low at the bow. Ideal trim is with the bow slightly higher than the stern (as checked along the tops of the main stringers ...which are horizontal when the boat is trimmed with the bow and stern the same height).
As far as stretching the boat to 28', this is described in the construction manuals already. We give instructions on how to shorten to about 25' and how to stretch to 28'. In a nutshell, all sections forward of amidships stay the same for any length boat. The construction manual gives new mold locations for the aft half of the boat to make the boat shorter or longer than 28'. You do have to place the molds (bottom panel assembly and sheer 'shelves' structure) in the new locations however and should not, for example, just run the side panels further aft past the last mold ...say if you made a late decision and had the molds in the locations for a shorter boat. The boat tapers a tad towards the stern, so just running the side panels (and bottom panels) longer would make the transom narrower. Either way, just follow the instructions in the manual and ask questions if you have them.
As for the curved transom that Ed in Australia has built, there is not problem with that as long as you keep the thicknesses the same. You will need to mount the motor (or jack plate etc) to the transom however, so you'll need to apply some thin 'wedges' that correct the curve and give a flat mounting surface for the motor. Ed is probably your best resource on this, and if I know him at all, I think you can expect him to be helpful ...I'm probably not going to re-draw plans and give instructions for a curved transom though. As far as performance and handling goes, the curved transom won't hurt anything, probably won't make a lot of difference in maneuverability (you won't notice), but may help a bit in seas that are trying to broach the boat. There's nothing wrong with the curved transom. I just didn't design it in and am pretty busy this summer... so you'd be more or less on your own with it (but Ed would likely help).
Brian
