I am sure the TX and FL governors will be responsible and award those seats to their growing urban populations.
For those who don't know, this cursed country can legally only have 435 House Reps. So when Montana gains a seat, it has to come from California or NY or one of the other populous states. And this will also effect the Electoral college by the same number of votes.
That's not even like a constitutional thing. Congress set that limit in 1929. They could change it at any point. The DC statehood bill actually has a clause permanently changing it to 436 instead of redistributing the existing 435 like was done for Alaska and Hawaii.