Only started a few days ago but most seems to be done.  However, I am still struggling with the first row describing the expedition.  The preamble says "letters in the first row must be replaced....".  Is this ALL letters or just some?  If just some - how many?
Are the replacements real letters or morse code?  How does this affect the down answers?  I really do not understand the reference to code options (six each) or 'blanks'.
A significant shove in the right direction would be greatly appreciated. Many thanks.