Posted for Ken Welter <rotary.coot@verizon.net>:

   I buy blank Northern radiators that have the tanks installed but no fittings or filler, I then make them to fit my application.
  The radiator I use is 18x18x2.5 core, they are welded with no glue so excellent for making custom units.
  If you are interested I have a 80,000 BTU fluidyne heat exchanger I would sell for $200, I tried it but overheated the water as I only had a 15x18x2.5 radiator and didn't want to modify the cowling for a bigger one.
  Ken Welter