I believe there is a mod you can do with Mustang rotors. Mustangs used a KH brake setup from the factory and had a larger bolt pattern. You have to machine the inside of the rotor for the Mopar sized bearings I think. The Ram Man was offering this service at one time but it was pretty expensive. I would not deal with him. I know there are posts about this on FABO. Maybe do a search there.
Also, I believe Wilwood sells a conversion kit that uses the stock drum spindles. This might be the large bolt pattern. Not sure.
Yes the early mustang rotor that is modified to go onto the SBP 10" drum spindle is the only affordable way to keep the track width the same in the front. There are links out there that show what needs to be machined, or buy the SSBC kit because that is the rotor used.
The big question to the OP is what are you going to do about the rear change over? ESPECIALLY if you are changing this in a Dart/ Valiant body, because the switch to LBP on the rear will also widen the rear track ... unless you can find an NS1/obsolete pair of 66-69 B body 10x 1.75 brake drums.
