To answer your 300hp question you left out the most important part, the step-up gear box inbetween the engine and drive. You simply can not spin the prop fast enough with the Merc reduction gears in the drives to go faster, the boat will always be slow 60 at best no more ever (my guess).
You need like we did to have Lenco (standard reduction box run backwards) or PatW (better choice) make you a box to go between the two. That is not easy, the box can bolt to bell housing easy and the short input shaft is supported by the drive plate and if extended the pilot bearing in the flywheel. The ouput of the box has to connect to the Merc drive input with a little drive shaft and probably need to run the staggered style carrier bearing and all so the shaft doesn't beat the drive to death.
It's been done but getting costly.
I think the QSD 4.2 is a better choice to try a drive hook-up, get 1.32 gears (or similar), the engine turns 3,800 rpm, 270hp or 320Hp and weight 1014 lbs - that could work again money is getting bigger and bigger for an old boat.
Look for used high ouput 250hp B series and start dreaming there first?