I know money is always the main concern but you could put in two of these but they are worth more than the boat itself http://www.cmdmarine.com/Products/Re...82062_6_08.pdf
If you were going to the trouble of converting to a single why would you not look for a used 250hp - 330hp Cummins B series engine? You would get the cruise you wanted and burn little to no fuel.
I would bet if you looked around you could fine a pair of 220hp or 250hp B's, they are 4 cyl and would drop into where the big blocks were. You could bolt these to a Bravo with no problem don't even go down the diesel torque issue as you won't prop it big enough to get the cruise speeds you want plus.
I know it's costly but would probably make you happier in the look run.