I am assuming that by the term "fixed-width ASCII" you are describing a file where all the records have the same length and the columns are aligned.
When I tried xlbo's suggestion (use .txt) in Excel 97, I got tab-delimited and varying record lengths.
When I tried "Formatted Text (Space delimited)(*.prn) I was able to get fixed length lines as long as the last one on the right was numeric (and not left- or center- justified). Strings in the right-hand column came out ragged and the record lengths were not consistent.
As xlbo says, the column widths need to be set verrrry carefully.
If this is a one-time data conversion, the .prn technique is ok, but for consistency and repeatability, I would look for another way.
Using a .csv file and formatting it with another program would be my first choice, but it depends on exactly what is going to be done with the output file, and you didn't tell us that. Perhaps the next step could use .csv format as its input?