Why not, he already had in mind he was going to seal him into it, so when he was on Gamabunta's back going towards the Kyubi, he already had the baby with him. Ready to perform the jutsu.

It's a sealing technique a different variation, why would Sarutobi want to seal Orochimaru into a living being, giving him the chance to come back to life? When he can seal him straight away into the Death God. Yes, I know the second part stuffs up the Kyubi, but maybe the Death God couldn't handle the Kyubi, or possibly it was easier to seal it into Naruto. I don't know this part, the technique hasn't been explained properly.