I would personally keep the BP you have now in the 75 gal, if he is already acclimated and content in that set up. There's no reason to move him to the 40 gallon honestly. Plus, I'm really kind of weird about swapping tanks and stuff regarding animals, I'm always scared about disenfecting and stuff like that. So I would leave the brand new tank for a new BP.
Also, I see now point in selling the 75 just to get a smaller tank... if your snake is already used to it and thriving, then what is the problem?