Agreed. Baloon tires rule! My long tail runs on 38-700 wich looks horribly fat in the eyes of road cyclists, but in fact I'm considering modifying the frame so it would take some really big tires. A 29" long tail it would be, and a much better bike for the cobble stones we have plenty of.
2. If carrying kids, I'd go with the schwalbe big apple tires (and build a frame in such a way as to provide enough clearance for these tire), as it'll give them a more cushy ride over the bumpy stuff.
That's something I'm unsure about.
3. I'd build a platform close to the CETMA. Make it a flat load floor that you can build a box on top. This allows you to take off the box when the kids get older so that you have a lighter and more capable cargo hauler.
a) the length - the CETMA and the other USA made box bikes seem to be shorter than the Euro made ones. I sense there is a (probably exagerated) concern with bike weight in the US (there is no similar concern about rider weight, it seems, but that's another subject alltogether). I think I could use a longer cargo bay but I stil need to make more drawings and tests with the kids.
b) is a platform really that useful/important? I imagine that being able to just throw things into the box and ride is a major advantage to any other arrangement that necessarily involves fastening of some sort.
4. I'd also build the "perimeter" frame for the platform. Run your tubes around the edge of the box rather than through the middle. It'll make your bike and platform stronger and save some weight.
This is new to me. Most home builders leave the cockpit intact (just because it's easier), wich makes the bike longer than it "needs" to be. I thought this was (only) a disadvantage. I'm stil undecided wich way I'll go. I'm drawing several sketches wich I'll discuss with the welder. He's a very nice and forthcoming guy. He'll help to decide wich design is more feasible with a certain 'cost frame'.
5. The space for the rider on the Joe box bike is really small, even the CETMA space is a bit short (if you are running a small stem). Be sure that you have a reasonable cockpit.