Mast position adjustements is in my experience a very individual thing and you pretty much have to try out for yourself what works. Fx, normally, you get the advice to push the mast foot forwards to easier keep the nose down in very windy conditions, but if anything, I rather push it back a hunch to get closer to the rig and get more control over the board with my feet instead (now we're talking really choppy and windy stuff). SO just go out and experiment. In practice, I tend to find a sweet spot for a board and then stay there regardless of sail size and wind strength. Usually in the middle of the box. Also remember you can trim the board a lot with boom height.
5.7 works OK on the Q76 but better with the 16cm front fins. But you will get a set of those with the 71. When the board get closer to being "over-sailed", you need to pay a bit more attention when just getting onto the plane. Basically, you have to bear off a bit to let the fins get some clean waterflow before putting pressure on them. Standard technique, but for some reason the board seem to "trick" some people into believing there is more grip from standstill that there is, resulting in a funny going sideways feel. It's a rare phenomenon, but I think you will recognize from the description above if it ever happens to you. But just get up to speed a bit more gently and all the normal grip is there again (also with a bigger sail).
On the 71: The 13cm fins will be a great addition. You gonna be able to do a whole lot of fin experimenting with two boards, three front sets and two rear sets. But unless with big sails and onshore, I think you will stick with the 13+11. But for sure try other combos. For me, its more about tuning the feel of the board than a pure adjustement to fit sail size. Again on the mast foot: stick it int he middle to start out with. My GUESS is that you rather would like to move it back in strong winds to soften up the ride and get a more "centered" and "ready" stance.