Do you already have the car that you plan to use? If so, the BS, BH thing isn't important. If you install a 426 into anything, it can be removed at any point. Just keep the original components to re-install later on. By the time you're finished, you will spend 20 grand installing a 426 into just about anything. That crate motor is missing a lot of stuff. Remember that you will need to change out the rear if it's smaller than a 3/4. That 20 grand you spend will make that car worth about 20 grand more. Been there...done that.
I'm assuming that you have either a 73 or 74 since you're talking about BS 318's.
The 426 will make the 70 and 71 BS cars worth even more since they're the only two years that a 426 would have been available .