I agree with George. Unless you have the Marti report or documentation to prove your 69 Mustang came with a 351C, then you do not have one. People often mix up casting numbers and casting dates. Manufactures did not switch and retool for the new model year over night. And the manufacturing year/model year is not the same as our calendar year. I do not believe any 69 Mustang ever came through with a 351C. Yes in 1970 an H-code 2 barrel could come through with a Windsor but not a M- code. These tales are often helped by early literature that was wrong and then continues to be misprinted. Just like the odd 68 GT 500 Shelby sneaking through with a 390 instead of a 428 or ordering a Galaxie with a 427 SOHC. Both legitimately dis-proven by their respective clubs and sanctioned judging associations, through research and actual manufacturer paperwork.
If your getting casting dates in the late months of 69 that is already well into 1970 production. Shelby's were considered late production and even them with casting dates starting in Aug and Sept of 67 are in 1968 model year cars. so a regular production Mustang with Nov/Dec 69 casting dates is definitely 1970 production.
Even now 2017 models of most manufacturers will be heading onto your local dealership lots fairly soon.