Basic rule of thumb is the length should be no less then 50% the length of the snake, so technically the 40 Gallon Long is I think 3 feet so it should be "ok" up to 72 inches but with a boa they are bulky so I would not want took keep him in there much past 5 - 5.5 feet.

If you are feeding moderately and your snake is a male (which seems to be the case) then I would say you will get a year or better with this tank as a go between.

That is just a guess so many things effect growth rates,