We were thinking of a 7' length of 6" Sched 40, slid into a 7'
lenght of 12" Sched 40, using either ribs welded to the outside
of the 6" pipe to keep it centered, or something similar.

Now the big issue...how to make a burner(s) that will uniformly
heat the inner pipe.

Do a search of the live steam sites, I seem to recall seeing
several designs for propane burners for railroad boilers that
were relatively long with a consistent flame along their
length. Think barbeque burner technology, where the fuel and
air are mixed, then released by a tube with slots or holes along
the length. I don't remember what there was for for baffles,
or how much the design of the surrounding boiler affected the
burner design, but it's a place to start.

--Glenn Lyford