Is there a difference between 1970 351c heads and 1973 351c heads? I have a 1973 351c with 2bbl heads and will be interested in converting to a 4bbl. I've been shopping on ebay and found 4bbl heads for a 1970 mustang and a 1973 mustang. I notiticed a difference in the area where the valves reach into cylinder (can't think of the name right now). I've read that Ford engines get their compression from the heads and not the pistons. My car is at 8.6:1 compression. If the 1970 heads will fit my car without and major modification, then that would be nice b/c I'm sure I could get a little higher compression.

My biggest question here is how much did the cleveland change between 1970 and 1973? and how do parts such as the heads interchange between those years?

 

Yes there are differences including chamber volume and valve sizes. Early 4v heads had large valves and small chambers. Early 2v heads had smaller valves and larger chambers. Later heads had larger chambers IIRC.
Regardless, they should all interchange, but different intake and exhaust manifolds may be required.