Need to make a reference grid for a letter size page

I am trying to generate a TIFF file with the following parameters:

 - sampled at 600dpi
 - letter size (8 1/2 x 11", or 5100 x 6600 pixels)
 - grid lines every 1000 units

The procedure I am using is this:

 - Run the PostScript file below through Distiller, converting it into
PDF
 - Ex****t the grid as a TIFF file from Acrobat.

The problem is that the resulting TIFF file has a dimension of
1700x2200, and the resolution is 200dpi.

I can change the resolution with IrfanView, but not the dimensions,
because the resampling will mess things up.

I guess I need to change the default (72dpi) resolution of PostScript
to 600dpi? How?

Any other suggested approach?

TIA,

-RFH

----------
newpath
% 5 Vertical Lines
  120   0  moveto
    0 792  rlineto
  240   0  moveto
    0 792  rlineto
  360   0  moveto
    0 792  rlineto
  480   0  moveto
    0 792  rlineto
  600   0  moveto
    0 792  rlineto
% 6 Horizontal Lines
    0 672  moveto
  612   0  rlineto
    0 552  moveto
  612   0  rlineto
    0 432  moveto
  612   0  rlineto
    0 312  moveto
  612   0  rlineto
    0 192  moveto
  612   0  rlineto
    0  72  moveto
  612   0  rlineto
% end of grid
  1 setlinewidth
stroke
showpage