is there any difference between 351C heads and 351M heads

[Eddy351]

yes the part number maybe. but the head is identical. The head flows good on the intake but have poor exhaust ports. Can be help by blending out the bumps.They will make less torque than a windser style head. Its hard to get decent comp. witout a dome and require canted style vavle releifs . If you want a cheap head than port early 351w head .It will make more torque and will be easy to get any comp. you want. The exhaust port on the cleveland killed the really good design of a semi hemispherical head. Its a way lesser version of a yates head.