The 351C was only produced from 1970 thru 1974 model years. Performance part development pretty much ended in 1980. Parts are hard to find and in some cxases expensive. Machinists are not in most cases familiar with them. The blocks were poorly cast and have no room to be overbored at all in some cases, best case you can go .030" oversize. The 351C weighs more than a 351W and is wider, making it harder to install headers and plugs. Cylinder head design favors high RPM power, OK for Nascar but not the best situation for street or drag strip performance in most cases.
The 351W is still available brand new from Ford. Speed parts are still in development for this overwhelmingly popular engine. There are ten or more manufacturers of speed parts offering aluminum heads for example. There are any number of sources for fabulous intake manifolds, roller rockers and headers. Every machinist has experience rebuilding one. They are dirt cheap at a junkyard for a rebuildable core. The bore can in most cases go as far as .060" oversize. The head design has evolved to the point that you can have great low end torque and pull all the way to 6500 in a streetable engine. Stroker kits are cheap, this engine can be had in displacements up to 430 CI.
C-Spot- Welcome, and listen to Royce! He is one of our highly respected resident experts!
On the W vs. C questions, if you do a search (and some poking and looking) you'll find several threads discussing the finer points of the advantages and disadvantages of these engines.
Enjoy this forum, you'll be hard pressed to find a better source of automotive knowledge and experience. Now that TCCN is gone, you WON'T find a better source of Cougar expertise! (There are other sites, but none of them offer the Q&A opportunities of this site)
Royce is correct (of course) but if you have a 351C it was at the time the performance small block. It was used in 70 up Mach1s, Eliminators, Panteras, BOSS 351s, Cobras, Cyclones etc. It was no slouch. Historically a "Cleveland" has value in whatever it was installed in. It also had higher compression (not so good on the today's gas) and poorer gas mileage. If you are not doing a #s matching car go with the Windsor.
The 351M is a different family of engines, shares only a few parts with the 351C. The 351M is a destroked 400 Ford engine. Both of these engines were made with the idea that all transmission bellhousings should fit the 429 / 460 pattern. The 351M has cylinder heads that are similar in pattern to the 351C and will bolt on but they are not performance engines and everything about them should be avoided unless you are lucky enough to own a 75 - 80 Fomoco product that came with one originally. These engines define the term "boat anchor".
While Royce is correct let me add that the 351m was brought into being because the 351 was already history and Ford found themselves in a jam because they couldnt produce enough 351's to meet demands. Since they already had the 400 that was introduced to be the sucessor to the FE motors that were having hard times meeting emissions standards of the day they simply destroked the 4" bore 400 to 3.5 to end up with a emissions leagal 351 that could be used in the full size car and truck lines of the day. It was 1977 if I'm not mistaken.
Both the 400 and 351m were never accepted well as they have the same basic block height and size of the 460 and weighed almost as much, with some really dismall HP ratings due to the really low compression ratios, retarted cam timings and such.
They do share the basic 351c designs and the heads are the same as 351 2v heads with maybe a valve size difference. The share the same oil pan, dist and such. The main differences are much larger cranks and journals, the formentioned bellhousing, dedicated motormounts and flywheels and dampners.
They are like Royce said to be not desirable unless you have a original car that had one of these motors and are needed for originality. The good news is since they do share some Cleavland parts there are some things that can be done to help them out, but why anyone would waste there time on them is a good question. Basically there regarded as Fords POS but they did serve there purpose at the time. mm
Just remember: the Original Elanor from the 70's Gone in Sixty Seconds was a Cleveland, and I personally consider it a good motor. A 73 351C asks for 91+ octane, so gas can get expensive; but any great obsession can be. And they're worth it. If it was a perf rebuild, I'd go with a low compression 351W, EFI'd and charged, turboed or blown; that can also get really expensive. But I garantee It'll be fast. As is, my H code can thrash a charged ricer in a 1/2 mi.