But even if the lengths are same, the resulting SHA1 will be different since you prefix the length before hashing

The only thing that prefixing the length makes difficult is using the same prefix multiple times: you basically have to make up your mind about the type and length before mounting the shattered attack. Also, the prefix means you have to do your own shattered attack and can't use the PDFs that google provided as proof of their project's success. Price tag for that seems to be 11k.

[1]: https://github.com/cr-marcstevens/sha1collisiondetection