Indeed, they probably have to have the same length since the length is inserted into the final block as part of SHA-1 hashing and every block after the collisio

Indeed, they probably have to have the same length since the length is inserted into the final block as part of SHA-1 hashing and every block after the collision must be the same. I don't think it's even possible to create a MD5 collision between two documents of different length yet.