Although I like the idea of the Toyota I think after 300bhp the wheel base of the Stratos will make it not so pleasant to drive on a daily basis and become a "point and squirt" machine (Neil will be the one to ask though).
If I were to fit a non Italian V6, I would head for a mid to well tuned Duratec 4 pot to lessen the rear end weight, to reduce the pendulum effect when chucking it about (having fun)....