TA-4Fs are a natural choice for the fast-FAC role, after all, the Corps used them in such a role (after TF-9Js and prior to OA-4Ms). You could even extrapolate later A-4M/OA-4Ms in service with the Army, or alternatively add A-7Ks to the A-7Ds for a fast FAC component.
For a FAC, I'd stick with something with a prop to be slow enough.
Regards,
John