Would the transmission bolt that hold the trans to the motor for a d series be the same length and size as the bolt for a b series trans? Really don't want to rummage around if the bolt can be interchangeable between each other.

 

Why not test them out and see?

 

the only thing i'm worry about if either one of them bolts are either longer or shorter than the other.

 

There's only one way to find out bro,just don't make them hella tight and you should know if they will work or not.
Or just get some from an old car at a junk yard.