Hi George, two boards is always better with the big range of sails you have - obviously a 3.7 is very different to a 5.3 on a board - so keep the 75 and go for a new Mamba 84, that should work well.
One board solution - for all conditions and sail sizes- Mamba 84 or Grip 86..if you get your weight down to 82 you could look at the Grip 82, but it's always nice to have a bit more volume, especially if you are not always in DTL conditions and strong wind/choppy big waves...bigger boards should be no problem in smaller waves/stronger winds...good luck!