The https://en.wikipedia.org/wiki/Shamir%27s_Secret_Sharing page is quite easy to understand, with a high level definition
Q: Are any 400 among the 1000 keys enough to reconstruct the BTC private key?
A: Yes, any 400 different keys are enough, whatever they are.
Q: Is the BTC key length a problem considering the current treasure keys length?
A: No problem, the reconstruction will result in exactly the right key.