I just bought a 1966 Mustang Coupe and the guy didn't know if its a 302 or 289 and he wasn't the original owner. Can you tell the difference between the two? and how? Thanks.
Your first clue will be in casting numbers on various parts of the engine. C = 1960's and the next digit will be a number representing the year the part was cast, e.g. C8OE would be 1968. This is not definite beacuse many of the parts are interchangeable, but if you have alot of C6 numbers, then it probably is a 66 289. They started making 302's in 1968. You can find a casting number for the block behind the starter.
Pull the starter off (only two bolts) and read the entire number cast into the block. There are many 302 blocks out there with 289 heads and intakes on them, so don't trust those numbers if the block is what you are concerned about.