As far as I know, the Palomino's were not really great overclockers in general. I had a 1500 that would do 1700 reliably, and a 1700 that would do 1900 (don't remember the actual GHz ratings).
Are you sure the board won't support Tbreds? The CPU compatibility list has the 8KHA+ rev1.3 shown as not supporting the Tbred, but the BIOS date for that is from December of 2001! The latest BIOS for the 8KHA+ is from May 2003.
The only thing with a board like that is that it probably doesn't support multipliers over 12.5x, at least not manually set ones. It should be able to read higher ones, but the option for over 12.5x just won't be in the BIOS.