Another method for getting track sag, is to drill a small hole into the hull side and insert a pin across the tracks and into the hull. This keeps the track down nicely. Make sure the pin is inbetween links and add a little pastel dirt on top to disguise. Two lengths of piano wire through both sides of the hull should do fine.
Another alternative is to use friul tracks ...... and get all that nice sag naturally. Im another who doesn´t really bother about exact detail ... and prefer to do something that appeals to me rather than be right, but friuls have to be the single best aftermarket item available. On Russian and german armor, they are hard to beat.