I've got my girl in a 40gal breeder tank. It's too big, but I know she'll grow into it. I got the tank before I knew that snakes are anxious about big spaces. She's gotten used to it; she spends a lot of time in her burrows or in the branches I set up for her.

How long have you had the snake? If it's only been a few weeks or a few months, I think it's better to wait. If you've had that snake for roughly as long as it's been alive, there shouldn't be any problem moving him sooner than later. Snakes enjoy cramped spaces, so I doubt the lack of room bothers him much. If the tank is too tall, maybe try building vertically. Add some fake branches or some pvc for your snake to climb on, that way he can move around all he likes.