When working with Fruils, what I usually do is as follows. I've found it works best for me.
1) Complete lower hull and temporarily or permanently affix all of the roadwheels, return rollers, idler and drive sprocket. Do not join permanently to upper hull.
2) Assemble the fruil track runs for both sides and mount, leaving that last pin unglued. This allows me to adjust for track sag, length, etc.
3) Dismount tracks. Complete model, and complete the tracks separately. Once everything is finished, join the two. I usually leave the last pin to be joined on the bottom run of the tracks, and obviously facing the inside of the hull. The fruils, when properly assembled, are extremely resilient to handling, and I've never had a run fall apart on me.