OK, here goes:
IN general, when tuning a certain shape to a twin fin, you do need to tweak it a bit for things to work perfectly. For example, it seems twin fins require slightly beefier rails around the footstrap area if you still want to be able to put power down in your turns. Often outline is tweaked too. When the ETs were first designed, they came from EVOs but rails are changed and outline tweaked so the board would still go vert well. But compared to many other boards I think the difference are maybe smaller, ie an EVO would work reasonable well as a twin. And the fact that you're a lightweight that rides a big board say that it might work better than if the board would have been relatively small.
If you like to make it a twin, I would place the boxes 13,5 cm apart (center to center). and put them so that the front of the fin boxes are right under the front screw of the back footstrap, possible a cm further forward (fine tuning depends a bit on which twin fins you're gonna use).
Another option is to convert to a thruster or even a quad. Then I would use the minituttle boxes in the front. They are easier to install than two new us-boxes for a twin conversion.
The thruster option (in my opinion) is maybe less interesting if this is gonna be your smallest board since (at least when I've don it) it has sort of powered the board up and made it feel bigger. Based on comparing quad and single (Quad 71 to EVO 70 - same shape, afaik) I would say the quad conversion tends to leave the "size feel" as is. With Quads I would go with future boxes for the rear. The EVO70=Quad 71 goes really well as a quad and I would imagine the 75 would do too. Tell me if you want some beseline measurement for a quad of thruster conversion.