Here's an update. Thanks to the good advice on this fourm, I've given up on the idea of putting water heaters above the boiler, there just isn't enough steel surface area to transfer a lot of heat to the water. I'm casting a refractory boiler out of plywood forms and I've gotten rid of the long firebox, it now looks to be about 24" by 24" by 30" long. The secondary combustion chamber will be the same dimension. Given my limited welding skills, instead I'm running about 100' of 1" black iron pipe through the secondary chamber for heat collection. These pipes run 18" horizontally, then turn back and forth six or 8 times to create a baffle system and let the turbulent air flow give up as much heat as possible to the pipes. I will also put in thin piece of steel behind the pipes to help the baffle effect.
I will start with about 10' of horizontal stovepipe before the chimney. Once I get a fire going and can measure the temperature of the exit gases, I will add or take away horizontal stovepipe until I have about 300F exit temperatures, to keep creosote from building up.
I'll still put some heat exchangers on the ceiling of the concrete building to preheat water before it passes through the boiler. I am also building a solar water heater that is big enough to supply my summer hot water. The OWB will supply my winter space heating needs. Both systems will share the heat sink of two 275 gallon hot water reservoirs, but will be run by two different circulating pumps. The OWB will require a 30 gpm pump whereas the solar collector will use a much smaller pump for a system of at most half a million BTU's per day.
I will keep everyone updated, it is never too late to give me the benefit of your good experience to keep me from doing extremely stupid mistakes. Thanks for all your input so far,you've prevented me from some serious stupidity.