Here was a site which talked about it some:
"We receive many phone calls from classic Mustang enthusiasts inquiring if the engine in their 1969 or 1970 Mustang or the 1969 or 1970 Mustang they are contemplating purchasing is a 351 V-8 Windsor or a 351 V-8 Cleveland. First of all, virtually all 1969 351 V-8 engines were Windsors and virtually all 1970 351 V-8 engines were Clevelands. There are a few exceptions. Over the years we have seen a few 1969 Mustangs with factory installed Cleveland engines. By checking the build dates of these Cleveland 1969’s we found that in every case these Mustangs were produced in the last week or two of 1969 production when perhaps the factory ran out of Windsor engines."