Here's the seatmap: http://www.seatguru.com/airlines/Bri..._747-400_B.php

39JK and and 40BC are indeed front row bulkhead seats. These have more space for your knees as no seat in front of you, but do have a wall in front, so you can't stretch your legs out under the seat in front.

You cannot have an emergency exit row as you need to be fully fit and able to assist the crew in an emergency should it be required. Clearly if you need sticks to walk, you would be unable to do this.

If you really need to be able to stretch your legs out, then I'd suggest that your current seats might NOT be ideal for you. The other alternative is to pay to upgrade to Premium Economy or even Business class.