There is no such thing as a 351 "modified" the M stands for the designation for 351 Michigan(plant). People just slapped the word modified on it cause it uses parts from the cleveland and the Windsor. The 351m is a work horse engine and is a DOG overall compared to the Cleveland and Windsor. So "you heard" wrong. I've owned and worked won them all.
Any 351 with decent heads and a cam (*edlebrock* ) will pull stumps all day long. 302s are for mustangs