I need to move a steam radiator about six feet from its current
location. This is a single pipe system with black pipe. The piping is
all accessible in the basement.

I have zero experience working with iron pipe, but I'm pretty good
with copper. I know that part of the boiler plumbing is copper (the
manifold coming out of the boiler).

Is there any reason that I couldn't go back to a convenient joint,
install a dielectric union, and finish the job in copper (with another
dielectric union at the radiator end)?

OK, I just went downstairs and took a look at the boiler installation.
Where the iron steam pipes connect to the copper manifold, they used
copper threaded adapters with no dielectric unions. Where the
condensate pipe attaches, they used compression fitting , which, I
guess could be an insulator, but since the steam pipes are tied
together, I'd have no way to check that.

So, is it possible that in a steam pipe, which is not under water all
the time like the condensate pipe is, that you don't need dielectric
isolation between iron and copper? This boiler was installed 24 years
ago, and there's no visible sign of corrosion at these joints.

I am aware of the slope requirements, and I have plenty of headroom to
do that. I don't want to use iron because, as I said, I have zero
experience with that. I don't have threading equipment, and even
though I'll measure twenty times before I cut once, I don't have much
confidence that everything will line up on the first try, and that's
yet another trip to the supplier to get yet another piece of pipe
threaded.