The easiest way is to just go into rom manager (free or premium), select download rom, go to Stock Images and you should see the .41 and .43 basebands. You need to install .41 first and then .43 after.
And oh my holy god... Don't do this. Avoid the .41 and .43 Basebands like the plague. I flashed them, my data service went in and out like a strobe light on crack... had to flash back.