It's going to be tough to hit right on because No one knows how well it's going to grip up, BUT
For Jr 1 Clone I'd start 21/63 expecting it to be a little low on rpm early and as track grips up it will come to you more after controlled practice, I'd have a 22 driver laying on the table If I had to drop rear to a 61 or lower I'd be on the 22.
Jr 2 champ I'd start 20/64 and keep a close eye on it up through practice.
There both based on the drivers maximizing momentum and ready to go fast.