the d15 isn't gonna be any stronger than your rebuilt z6. you can get tuner toys rods for like 225 and vitara pistons for ~100 w/ rings. get your block honed and the pistons pressed on the rods and throw it all together. it'll put you a bit over the cost of a d15, but it will be 10x stronger. if you don't like that idea, then just pull your d6 apart, get it honed and put new rings in it. don't bother with another engine.