I converted a jp few 111L into a quad, now it's a frikken awesome light wind wave board. Tail shape is ok but ideally I would like thinner rails with more bottom tuck in the tail.

why not put thruster boxes in first, then try it, if its good leave it otherwise if its still not good, then chop the tail.

Strange mix of ideas though. Pulling the tail in with wingers will narrow it for a better tighter carve and improve top turn - yes.
But a swallow is wider in the last few inches so you can't cut a swallow into an existing rounded pin (or even thumb tail) very well as its already a bit too narrow. In trying to get the width between points, you may shorten it too.

So I'd just take 1/4" off each side from a winger starting at 1ft as you said, and not try a swallow.

But a good point from Madge that a thruster conversion alone may do what you want.

I love this experimenting, and old FSW that I converted to trailer ended up magnificent.

The 30cm point the tail,width is 39.5. Reducing by using 2 wingers on the rails, decreases the width by around 2cm.
37.5cm . 38.2cm width is an average of 5 boards and very near the 91JP FS, 6 years newer.

plotting this out affords quite a few tail shapes.

forceten said..
Thanks, did you leave the existing box, probably powerbox ?
the freestyle shape is different to yours.

Oops sorry misread your post. Yes left the power box, but haven't used it since the mod.