You can build a "clone car" of the car to be represented. So yes, you can use an ordinary 289 car, and build it like a 427 car. You do not have to start with an actual 427 car.

There are certain car/engine combo's that are favorable, or more competitive.

You can use another FE block asa long as the bore size and stroke do not go "out of bounds" from the NHRA acceptable limits.
After all, the cross bolts and side oil galleys dont make it any faster, just stronger.

You could run a K code small block also if you can prove that any were made. I'd be willing to bet that there were indeed a handful of Fairlanes built in 66 with a K code, but they would also be rare and few.