Re: 351 m ??
I will try to shed some light on this one. If anyone else has any other insights, please feel free to chime in...
First, the 351w = 351 Windsor small block. this is the 351 you typically find in most trucks from about '69 to '96, as well as other applications in rear wheel drive vehicles. As far as I am aware, this is the only moter that made it to the Fuel injection days beginning in the 80's. This is basically the same block as the typical 289, 302, and 5.0L motors, with many parts (such as the coveted heads and intake) being interchangeable.
Next, the 351c = 351 Cleavland. I believe this is also a small block. The big difference is the is the upper end of the motor. If I recall, it either had overhead cams or 4v per cylinder, I can't recall exactly. These were rather powerful motors generally used for performance applications like in some Boss Mustangs, ect. There were also 302 Cleavland motors as well. The parts for these blocks can be expensive and ARE NOT interchangeable with the windsor block 289/302/351.
Finally the 351m = 351 modified. This is basically a de-stroked 400 big block, hence the "modified". these were more common in some F-series and broncos in the 70's. I think it came in some of the better performance packages like the tall-boy F-250 and such. I don't know too much about these except that I have heard of them having problems overheating. If you use this motor, consider getting a larger radiator.
Basically, as these blocks go, the more rare and expensive the parts equates to better performance. But don't expect to find Cleavland block parts at the parts store in the small town by your favorite mudding hole.
i hope this sheds some light on things.