Leaving the original sleepers results in something too small for 1:24 scale. You need to add new timbers to the correct scale and spike them into the existing Flextrack. Using every fourth space equates approximately to 30 inches which is comparable to a lot of narrow gauge track. By using the new timbers and leaving the plastic in place until the track is finally glued down, you will find the plastic hangs above the surface and can be removed quickly once the glue is hard.
A tale of two turnouts (before and after):
Flextrack being redesigned:
Closeup of the finished turnout showing that only the sleepers that support essential parts of the mechanism are retained, the rest are discarded. The original plastic parts are padded with timber to create the look of 1:24 scale track. Be especially careful to still support all the pivot points and the throw mechanism.
It doesn't take too long in the whole story of your Gn15 universe and I think creates a far better visual impression than making do with 0-16.5 crazy track or hacked about HO track. So far I have created around 10 metres of track for my layout and approximately seven turnouts.