Where did you get them and what was the part #?

Smaller than the factory 11.3" diameter won't work, as the friction material will hang over the edge.

 

they gave you the wrong part.
i got mine from Napa and they were the same size as the originals

https://www.napaonline.com/en/p/NBO4885533?partTypeName=Brake+Rotor+-+Front&keywordInput=brake+rotor

 

It's hard to imagine a 10 inch rotor that would have an integral hub and 5 lug studs.

 

None are 10".
Exact measure is 11 19/64"--Factory specs.
I'd say a error on listing? Anyway if not correct, return. None of the new rotors will be a 2 piece, all are one piece turned rotors.
Then there was a question about the 68 69 rotors. The 68-73 rotors are same---Now wait a bit here, they are all the same, the only difference is the races. Insert a 70 race, you got a 70-73 rotor. Knock out that 70 race, put in a 68/69 race, you have a 68/69 rotor.

 

Maybe you got a Mustang II Mach1 Rotor... 9.2 inches. LOL.