that´s quite quiver! thanks for the brand loyalty on the whole range of the Hawks.
So let´s look at the widths, generally the Blast´s are fairly narrow vs classic freeride boards, but due to the parallel outline, you get a lot of power, acceleration drive, whilst a lower rocker line also helps to control that power etc.
You want to replace your 62/68cm wide boards with the Blast 115 which is 66cm wide. I would say yes, not a problem, the 115 should cover that gap quite nicely and certainly works with a huge range of conditions, I have sailed with 8.4 Warps and under 5sqm Hero wavesails, so from 12/15 knots to well over 30knots. My Daughter´s favourite board and likewise a few friends love it.
Some Hawk owners take a bit of an adjustment to the super easy feeling, whereas the Hawk tends to lift and sail very much on the back foot, quite nose high, the Blast sits quite flat in the water and absorbs the chop at a much better angle. Is easily as fast on flat water and in chop considerably better. Turning wise, no comparison, the Blast is considerably better. Perhaps you may need a session or two to adapt, but it is certainly worth it, the Blast is now going unchanged into it´s 4th year, showing how far ahead of the pack that concept was and still is!
Yes, you would need to get an older version for powerbox, but generally the production fin works for a huge variety of fins, I never use the same fins in different boards, so perhaps only need to invest in one extra tuttle fin if needed, I would go for the newer version, always better for resale value.