Got any pictures of how the cage ties into the framehorns? Sounds like your engine cradle uses the standard subie engine mount locations?
Is it a Subie gearbox?
I've never had to set a car up for drag racing so unsure if stiffer or softer springs would help keep the tyres on the floor. I mean the only thing trying to stop the rotational force of the tyre is the spring and damper along with the vehicle weight and weight transfer. Softer springs and damping would allow for more weight transfer and then dual adjusting dampers so the rebound is softer? Just some thoughts.