CZ 550 Standard lenght firing pin works fine, CZ 550 cocking piece shroud also, I did somthing similar but the other way arround, firing pin and cocking piece shroud from a ZKK 600 30-06 to a CZ 550 9.3x62.

Hope this helps.

ETA In fact I also mixed firing pin from CZ 550 9.3x62 with Cocking piece shroud from ZZK 600 and it also worked perfectly.