Not wanting to disrespect Bruno but I find it hard to imagine him beating any of the top 20 ATG heavyweight champs.
No way does any version of Bruno beat Holmes. In fact, I’d probably pick the 42 year old Holmes over the best version of Frank. The key difference in their mental make up was that Holmes consistently found a way out of trouble when he was put under pressure and could battle through it whereas Bruno found a way to cave even when he was on top. I think Frank might have his moments, but that’s all they would be - moments. I’d be cheering for Frank all the way, though.
Holmes stops him. Bruno was powerful, but Spinks was a lot faster, a lot more skilled, and had great stamina. Spinks outhustled Holmes, Bruno wouldn't be able to do that.