It makes sense that Excel won't save all 750 columns  in a 2003 format
- since the 2003 format  won't allow for more than 256 columns.

Your attempt to use Excel 2007 would be the first thought I would have
as well for 750 columns. 38 Mb would be a bit frustrating.

The only other options that come to mind would be to...
1. Present your 750 columns vertically instead of horizontally.
(Probably easier said than done.) or
2. Use three worksheets - showing 250 columns per sheet.