[Scribus] Custom paper sizes

[ephemeron]

I created a layout on a "custom" paper size, 5.75 x 7.75.  Now I
want to print it on the actual paper, which measures 5.75 x
7.75.  How do I do it? 

Exporting to PDF produces a page of the right virtual size.  But
when the page is sent to the printer, the printout begins some
way down the page (as if Scribus is trying to center the layout
on the preset size for, let's say, A4 paper).  Is there a way to
force Scribus to print from the top left edge?  This seems to me
to be the right thing to do when custom page sizes are involved
since, as far as the printer is concerned, paper sizes differ
only from the bottom right edge.