Dermot, thank you, this was the option that I was looking for. I am trying to do things as they should be done.
Unfortunately, it doesn't seem to work. I tested my shallow arc (polyline solid made out of box, with lower pline converted to arc), and it did nothing. It may be that arc is to shallow (15 ft. span, 1ft bow), but I will test that from the start.
On the other side, Perry's method gave instant results, in spite of the fact that 100 segments are a bit extreme. And there was no problem to turn that into one nasty 3D molding polyline...
That's just the way I am - extreme all the way.
Boxon1's post in Arcs look broken was marked as the answer