Cossie Dave's standard crank bent at 701bhp so perhaps keeping it in the low 600s might be ok. After that I would go for something new and as Stevie says why not go stroker if going to new block and new crank etc.

Just took my car out for a blat and it was wheel spinning in second at 4000rpm on high boost as the roads are cold and damp. At one point I was sure I was in third due to how hard it was pulling, it was in 5th and I had no more gears to go...that's with 469bhp, makes me wonder what it would be like at 600 and more torque of a stroker.