Printing cut off when printing on a small form

I'm using Word 2000 with an HP LaserJet 4M Plus and Windows XP's standard
driver for the 4M Plus, and I have encountered a baffling page layout
problem. I'm going to explain this as clearly as I can, but you will
probably have to make a drawing from my description to understand it!

I'm trying to print an address on a United States Customs form 2976. This
form is 7.5" wide and 3" high. Obviously it had to go in the printer
lengthwise and be printed in landscape mode.

I first tried setting the page size to the actual size of the form, but Word
just printed blank pages, for reasons I do not understand. Therefore I set
the paper to letter size and adjusted the top and left margins to position
the address on the form.

When the 4M Plus prints in landscape mode the leading edge of the paper
becomes the right edge of the printout, so I had to feed the form right edge
first and adjust the page margins to print the address near the upper right
corner of the virtual (letter size) page.

I arrived at the proper margin sizes by printing the address on a sheet of
letter size scrap paper. I then superimposed the form on the scrap paper,
measured the positioning error, adjusted the margins, and tried again until
I got it right. The top and left margins were set to 1.6" and 8.4",
respectively. The address was positioned to appear about 1.25" from the top
of the form (the left edge, as it goes in the printer), and 4.8" from the
left edge (the trailing edge).

Then I tried printing on the form. It came out with the address perfectly
positioned, except that the left 0.4" or so of the address did not print.
The remainder of the address was not offset to the left. It's as though I
had tried to print outside the printer's margins, except that the
non-printing area was in the interior of the page, not at the edge.

I tried printing on another letter-size sheet. Again, it worked perfectly.

What is happening, and how can I fix it?