Bruno might actually lose this. I can seem him coming out pumped up, getting his jab going and brutalizing Valuev a bit, but I can't picture him stopping Valuev, and then as the fight wears on he'd get tired, physically and mentally. Probably Valuev wins in the last round.
Bruno should easily take Valuev. Then again Bruno may become frightened by Valuev's size and fade late in the fight. To have such an imposing physical presence, Bruno had an equally fragile mindset.
Frank Bruno employed Mike "The Giant" White to spar with him back in the day so he wasn't totally unaccustomed to boxing with men who around 7 foot and north of 250.. Looking at how painfully slow Nikolay looked against many of his foes, I don't think Bruno would have to work hard to the point of gassing.. I think Frank could win on points.
Bruno on points is most likely, but an upset if Bruno tires or gets caught can happen. Gassing and confidence were problems with Bruno.
Bruno on points in a hard fight. Could see Bruno piling up the points with his excellent jab and Valuev taking the later rounds as Bruno tires. 60/40 fight in favour of Bruno I'd say but wouldn't count Valuev out.