Wouldn’t brazing be the normally preferred method of joining copper
to steel? Also, brazing is good at gap filling, and it’s difficult to
get a really close joint on a large sculpture. Of course, the brass
might show if the joint is visible on the finished piece.
You didn’t say so, but I suspect you are having trouble raising the
temperature of the large copper pieces to the necessary level. A
large oxyacetylene or oxypropane torch should work, or Harbor Freight
has a massive propane torch (actually marketed as a weed burner or
driveway ice melter) for $20 which is great on large pieces.
You might also use firebrick to create a makeshift kiln to help hold
the heat while soldering your piece. (You’re probably already hip to
this if you often work on large pieces).