I want to find a way to make the group column header print across the report,
instead of being as wide as the column itself.
In a multi-column report?
You can't.
What you can do is size the report width to the full width of the
paper (less left and right margins).
Place the labels that normally you would place in the group header in
the Page header.
Repeat them across the page for each column.
Click on file + Page Setup + Columns
Uncheck the Same as Detail Check box.
Manually enter the wanted column width.
Run the report.
Adjust the spacing and placement of the page header labels.