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Cap, the 351m was a much better engine than either the 351c or 351w...you're probably thinking of the windsor engine which most car enthusiasts did agree was a pos.
You never ran a 429 boss did you? especially in a 69 fastback 'stang...ya thinking about the 429 that got nerfed because someone said it was too hot for the average mom and pop car?
Sorry, but you are way wrong about the 351M. I worked at Ford during the 'M' fiascos. Yes, the 400M was also a piece of junk. A couple of the worst engines ever made. The 351C and 351W are much better engines. The 351W is a popular engine for hot rods today.
The confusion about the 351W centers around the three different block Ford made. The first generation 351W block was a damn strong block and the preferred block to be used today for racers. It has enough meat in the crank webs to machine for 4 bolts caps.
The second generation 351W block was too weak. Ford stripped it to make it lighter for street duty. For the street it was fine, but it cannot be used for any serious performance work.
The last generation 351W is the best one yet. Lighter than the original, but stronger, it is still for sale under the 'Sportsman' name.
Show me a 351M running today, and I will show you an engine that is dead but does not know it yet. The design was flawed from the start. Not much about the 'M' family is good at all and there is a reason you will not find an 'M' engine, or any of its parts, anywhere in the Ford performance parts catalogs.
The rumor about the Boss 429 being too hot for the mom and pop mobile is just that. Rumor. Ford killed that engine because they did not build it and it was not a very good design. It was meant to help them keep up with Chevy's 427, but the year it came out, Chevy upped the ante again and still killed Ford. Ford felt they could not wring anymore out of the engine so they killed it.
The big problem was the lousy head design. It was just bad. Poor castings along with too many impurities in the aluminum was just the start of the problems with these heads.