I’ve been told this engine experiences engine block cracking pre 1977. Problem is in the water jacket above the lifter bores? Can anyone shed some light on this problem and can it be detected without tearing apart the engine ? My engine is in a 1976 Cougar XR7. Thanks to all who can help me out.
You might try one of the 351 Cleveland sites, like Cleveland’s Forever http://351c.net/board/index.php
Thanks for the direction. 
A 351M/400M is not the same as a 351C. I do not know if you will find much information on a 351M on a 351C site. You might try a Ford truck site as a lot of those Ms found their way into the late 1970 F series trucks.
Randy Goodling
CCOA #95
George Reid’s book “FORD 351 Cleveland Engine - How to Build for Max Performance” covers the 351C, and its evolution to the 400 and 351M. It does note that “Early Cleveland blocks seem to have been plagued with cracking issues in the lifter valley.” For 351C he’s recommending the D2AE-CA casting (1971). The “early” 351C’s appear to be D0AE castings (1970 - 1971). The 400 and 351M are D1AE and later castings. I’m sure there is much more to the story than that, but that’s all that is apparent from the book.
I’d try to find some comparative numbers of the total reported cracked blocks vs total blocks built. If the percentage is fairly low, then you may be worrying needlessly.