I've experienced this a few times myself on various Toyota applications. I usually place something up against the inner sleeve, like a wrench, and then hammer on the side of the wrench to transfer force to the sleeve to spread it out. If all else fails, remove the bracket, insert the bolt through the fixed hole, but stop half way between the ears, thread the nut a few turns onto the end of the bolt and then continue the bolt through where it (via the nut) will stop at the sleeve. Then lightly tap on the bolt head to transmit force through the bolt, nut, moving the sleeve.
Kudos on getting a reman Denso alternator and not some crappy non OEM unit
BigMike