Depends what you mean by dash. If you just mean the instrument binnacle, instruments and warning light panel then that is fairly simple. See here:
Installing a Defender Td5 Instrument Pack into a 200Tdi/300Tdi - Land Rover Technical Blog
If you want to swap the whole dash for the post-2002 affair then that is more involved due to the moving of various electrical switches etc. but it's nothing insurmountable.
wiltshire: The Td5 instruments are properly backlit like a modern car, so one can actually see them at night properly. And being electronic the speedo needle doesn't wobble about like the old one does when the drive cable wears. Plus the LED warning lights are far more visible and just look 100x better.