Hi Ruf, and welcome to RRR from northern Alberta, Canada. I also thought like Sam Fear did, that you were from the far away OZ. I have a friend that collects 1939 to 1953 Dodge and Fargo trucks and he swears that flat head sixes don't seize up much. Yours may not be that bad to get turning again. I just took one apart after some time with penetrating oil in the cylinders and the pistons moved again. Not all of the valves moved though so I have to manually free them up.
I'm looking forward to seeing the Desoto truck as we don't have them here.
Keep on tinkering.