Congratulations with the addition to the collection.
In general I'd do the same what Thamnophis suggests. It's better to acquire unrelated bloodlines if that's possible. There is however a lot of inbreeding with morphs, but I'm still not convinced that this will not give problems in the long run (so for future generations).
What rick means is indeed that 66% of the regular looking snakes ( which are 75% of the young ) are het. But there is no way to tell if the snake is het or not, there will be no distinctive mark between the regular looking young ones.