The M block cam wont fit in a 302, but will work in a Cleveland.
75-79 351M cam : lift specs: .407/.407
77-79 351 M truck: lift specs: .433/.433
71-79 400 M: lift specs: .427/.433
77-79 400 M truck: lift specs: .433/.433
If you want to go with a factory cam, I recommend using a 73-74 4 bbl Cleveland hydrualif cam: lift specs: .480/.490.
the M block heads can be used on a 302, but you will need to use Boss 302 style pistons, which use a 5.155 rod vs the standard 5.09 302 rod. There are manufacturers that make boss type pistons for use on the standard 5.09 rod; Keith Black being one of them.
The intakes wont fit the 302, as they are too wide, so you will have to find a B&A Performance unit to work with the 2 bbl Cleveland head which is what is on the 351/ 400 M block.
There are a lot better cams to be used in an M block than the old cleveland cams.
One other thing. The cleveland head runs a 1.73 rocker arm where the 302 uses a 1.6 rocker arm ( excluding late model HO 302 or 5 Liter). This will increase your lift by a hair over 8%, but has no effect on duration.
IE: a cam with a .450 lift in a 302 with stock rocker arms will have a .487 lift with the cleveland or M block rocker arm.
The cleveland rocker arms can not be used on the Winsor head without relocating the rocker arms mounting point.
The above mentioned 73-74 cleveland 4 bbl cam has a duration of 206/221 degrees @ .050". It would probably liven up either a 351 or 400 M quite a bit, as both of their stock cams are below 200 degrees duration @ .050".
Other nice things to know. Clevelands, M blocks, and Limas all use the same distributor.
A cleveland intake will not fit an M block with out spacer plates due to the difference in deck heights.
Cleveland averages 9.2" deck height.
351 & 400 M average 10.3 " deck height.
Front timing covers are the same.
In 76-77 only, in California only, 400s were sold from the factory with a 4 bbl intake.
While rare, would be a cheap hop up item I am sure, with a decent carburetor, say in the neighborhood of a 600 CFM.
Edelbrock and Weiand both offer street intakes for the M block.