351 M block
There is nothing wrong with the M block, as was mentioned, they, along with the 400, have gotten a bad rap because they were produced during the smog era and theres not a lot out there available for them.
While they do share some charactoristics with the Cleveland, such as having common heads, exhaust, camshafts, timing assembly, and distributor, the M block is 1.10 inches taller in deck height and about 25 lbs. heavier. The M block intake is wider because of this.
Wieand and Edelbrock both make aftermarket intakes for the M block, which are rated for 0-5500-6000 RPM.
They do not share common crankshafts, the M block having 3 inch main journals vs 2.75 inch, for the Cleveland. You can put a Cleveland crank in an M block by using spacers in the mains, as is done with crankshaft swaps in some of the Small Block Chevies.
To switch to a Cleveland, or a Winsor, you will have to use a different transmission as the M block shares it bellhousing bolt pattern with the Lima Block (429/460). The M block also uses the same engine mounts as the Lima.
Carburetion choice, as I see it is pretty much up to the individual. While I do have a preferance for Holley carbs, there is nothing wrong with the Carter/Edelbrock carbs, just dont have that much experience with them.
The stock cam in the 351M is the same grind as was used in the 2 bbl cleveland block, and is pretty mild. The 400 cam is slightly larger, and very close in grind to the 4 bbl hydraulic cleveland cam.
I would recommend going with a cam that has a wide lobe separation, at least 110-112 degrees. Crane has a variety of these in the Power Max series.
The wider lobe separation, keeps the cylinder pressure higher at lower and mid RPM, which would be a definate advantage for street driving. I would also stay below 210 degrees duration as well.
You didnt mention what year your 351 M is, so its hard to know the advertised compression ratio. Gettting your compression up around 9-9.5:1 would be a help in both fuel mileage and a small boost in overall power as well. and you should be able to run it quite easily on 89 octane gas.
For timing, I would recommend starting at 12 degrees initial, and if your stock distributor has points, increase the centrifical advance as well.
This is done by removing the breaker plate and pulling out the centrifical stop rod plate on the shaft and turning it 180 degrees and reassembling. The two posts will have a number stamped next to them. One may have a number like 11 and the other will be something like 13 or 14. Set the stop post plate so that your distributor is using the one with the higher number. You can also pick up a degree or two of centrifical advance by removing the little rubber bumper off of the post.
You should come up with a centrifical advance of around 26-28 degrees, and thisw with the 12 degrees initial will put you at 38-40 degrees total advance. If you have access to some other Ford point distributors, see if you can rob the counterweight springs out of them. Ford used 3 different tensions of springs, at least, that I know of. Putting in two of the lightest springs will give you full advance at around 3000 rpm. These simple mods on your stock distributor wont put it in the same league as a Mallory or Accel, but since you are dealing with a basically stock setup, it will definately improve your performance, and help your gas mileage as well.
This distributor trick can also be used on any of the Ford Electronic ignitions that use centrifical weights for advance.
2 bbl.Cleveland headers will bolt onto your M block since they use the same head, and you probably wont run into any block clearance problems due to the higher deck height of the M block. You may however, run into firewall clearance problems. IF you dont have headers yet, and are thinking about using them, contact the header maker of your choice and discuss clearance problems.
I would recommend not going larger than a 600 CFM carburetor, and if you dont plan to turn the engine over 5000 RPM, a 500 CFM would do the job quite well, and make it a vacuum secondary carb, so you can adjust the RPM at which the secondaries start coming open. There are a lot of other things you can do to your M block to improve its liveability as well as enhance performance. Send me a PM and Ill give you a few more tips on it.