2V Cleveland heads vs 351M heads

> Is there any significant difference between the earlier 2V open heads and the later 351M heads?

Yes. Some of the later 351M heads have a more restrictive exhaust port with less
porting potential (water passages wrap around the exhaust valve guides). There were
also some variations in chamber volume. Aussie 302C heads will provide a compression
boost and have more material on the exhaust face that allows sealing against 351C-4V
sized headers.

Dan Jones