With the seat all the way back, The back of the seat should be touching the rear wall of the cab. If it does, there is no way to increase the legroom--other than section the cab and add 6 to 8 inches.
If the rear of the seat does not touch the back of the cab, then clean out the accumulated junk behind the seat.

__________________
1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags