I was generating some seeds with your mod enabled and found that each time I regenerate a world with the same settings, ruins were almost at the same positions but were never the same.

And even reloading the same save doesn't generate the same ruins ...

Unfortunately, I cannot do this, due to the way that I generate ruins. Ruins are pre-created setups of entities that have location/size restrictions (such as not being on water), and the entities that make up the ruins are spawned with a series of spawn commands, that are triggered when a chunk generates. They aren't something simple like a single entity that could be fed into the game's autoplace.

I could possibly set the random seed to the world seed, which might do as you propose. I'll look into it.