I have him at 5. You could argue he overtakes Holmes at 4 if he pulled it off. On the other hand HW rankings are very subjective. Most people have Ali and Louis at 1 and 2 but I'd be confident Wlad and Lewis beat them both H2H and maybe Holmes does as well.
If he became heavyweight champion at this stage which would mean beating a contender + Uysk at over 50 years old ? 1st IMO.
I have him at 5 now, so it would boost him to the 3rd place above Holmes and Lewis. Not sure if he would be the GOAT level like Ali and Louis.