- Long frame is not crazy, or really worth getting all exercised about. It obviously isn't conventional, but this kind of 1.5 frame has plenty of track record. You probably need to square the % increase to get the weight increase, but it is not going to be all that great, and makes for a real Cadillac ride.

- If you really mostly need the extra load capacity to carry 20 gallons of water, that just scream trailer, if you can find a practical one that will carry that much. Unfortunately stuff like the Bob is limited to 30 kg.

- 48 spoke wheels are nice, I would go for them. But I have been 340 pounds loaded on the bike, and had no problems with 36 spoke wheels, 32 spoke 26" wheels. More spokes are better. soldered spokes are not a good idea. You need to be able to wrench on the wheel, and a lot of spokes should deal with load issues. What value if any that soldering produces is open to question, so I would drop that as just another out there idea.

- I don't see any reasonable way to make all this packable, but if you can see your way to there, good ho.

Have you talked with any frame builders who might want to do this? Sadly such bikes often overshoot available tooling. When I got my anvil jig, I thought for 4 K it would handle bikes like this, which I wanted to make, but no such luck.