If you want more base power. I'd go with the A6 block and Y8 head. You'll have good power all around, and make a little more then a stock Y8.

If you want to boost the motor, I'd stick with a Z6 head and some 9:1 pistons with Eagle rods. With a good tuner *cough* MTBER *cough* you'll make some fun and very useable power.