Nice links Henry, but the fact that Ford and Chrysler were given/sold 1963 MK II Mystery Engines is just too much of a coincidence that the 335 and 385 ended up as they did. The cylinder head design doesn't follow any previous Ford production or racing Engine. Junior Johnson, who raced many Fords and Chevy's confirms this, he was there, and very involved.
Obviously not carbon copy's, but the exact same cylinder head design layout.
Right down to the MK II lifter banks which were later up dated with the 1965 MK IV.
But this doesn't take anything away from the fine performing 335, it just came at the wrong time and lived a short life, 1970-74. Boss 351 was a killer Engine, just not many made, and expensive to replicate.
They always were a light weight design and not as durable as say a 351 Windsor.
The 400 and later 351 M cylinder heads followed the same design but were severe casualty's of emissions etc and even lighter/weaker in construction.
Having grown up in the 60's and very involved of many aspects of automobile sales and repairs I can personally and anecdotally confirm the 351C, 351M and 400 were weak in all durability aspects. Many blew up badly, cores were very scarce.
Chevy, Mopar, even Ford 302-351 W Small block Engines were much more plentiful, interchangeable and rarely blew up.
Fords have lots of power potential as evidenced by their success in Engine Masters.
The 385 was a much more durable Engine that also was very powerful at first, but was also a casualty of emissions, etc.
Unfortunately the Chevy MK IV suffered from emissions, even more than the 385, by 1974 a stock 460 was usually considerably more powerful than a stock 454.
Ford has enjoyed much success with their OHC Engines, G.M. not so much.
Luckily the various LS versions are cheap, light, small, durable and fit Mustangs!!