The very cheapest solution is to grind off all the threads but the outer 3 rows from a 24 TPI bottom bracket cartridge. $20 for a UN54 BB and a little work with a file, and you haven't done any modification to the frame. It will screw right in, and hold just fine without creaking or loosening.

When you re-thread a 26 TPI shell to 24 TPI, you only end up with about 3 rows of good threads anyway. Might as well take the material from the replaceable unit (the cartridge BB) rather than the irreplaceable part (the frame).