You are all just a little off. I've been researching the 335 engines for over 25 years. The 335 was in the design house by '68. They "borrowed" the heads in '69 to build the 302 boss for scca racing. You might say that the the 302 boss was the first production 335 series engine although the bottom end was the "w" design.
The 351c was introduced in '70. The "c" was built several different ways including 2bbl,4bbl,HO,CJ. The last year of the Cleveland was '74 (I have one,I also have a '70).
The 400 was introduced in '71. '78 was the last year for a car.
The 351M was introduced in '75 as a way to simplify production and reduce parts overhead as nearly everything interchanged with the 400.
The biggest problem with the 351m/400 was the timing chain which retarded cam timing. By doing nothing more than replacing the chain with a good aftermarket racing style chain you could make it a whole new motor. The heads on the 2bblc and 351m/400 are identical. the valves and ports are bigger than an all out racing sbc. The valve/ports are too big to present any velocity. Velocity is what a head needs to make to create low end torque and fuel mileage. Don't get me wrong, I love this engine.
The austalian 351c heads were identical to the ones here but, They built a 302c (destroked 351c) which used the closed chamber 2bbl heads.
Yes I do have writen proof of everything I wrote.