The fins on AT kits can go in pretty tight - it helps to have an expended motor in the tube, and to have the BT in some sort of cradle so you have something solid to push against. I'll sometimes test fit the fins in the fin locks on the motor tube, without the main BT, they go in a bit easier the 2nd time.
This is a great sport scale kit (I've built 3), and yours looks good.