Orson, yes the obstacle cells receive 6 of the extra letters.
Smithsax, the letters used for obstacle cells (numbering rows top to bottom) : row 4 M, row 5 L, row 8 BP, row 9 G and row 11 C. There are 6 across targets (2 are on the same row) : 9, 5, 5, 8, 6, 6 (the 5th of those in reverse). The other 4 targets are in columns : 6,4,7,10