now im looking at my Mustang Recognition Guide for 1965-1973 and it says:
that in 1968 a total of 9 motors they were
289 4 - barrel
289 2 - barrel and only until mid year then it was replaced by the 302 2 - barrel
302 4 - barrel
427 with 390 horsepower
in late november of 1967 they offered the 250 6 cylinder and in january you could get the 428 cobra jet
it also shows a 390 as a option
the first time it brings up the cleveland is in the 1970 sections and it reads:
A new 351 Cleveland 4 - barrel engine entered the Mustang line up in 1970, featuring canted valves (like the boss 302) and 300 horsepower, replacing the deleted 351 windsor 4 - barrel.
Hope that helps. Now I have found a few things the book isnt right on such as the mach 1 codes in the 69 and 70 it says a 5 in the vin means mach 1 and no 1969 car had the 5 in the right vin location they all had a 2. So I think this book is 99.9% correct.