For 250-300 i would run your stock block. For anything over i would upgrade the following, pistons, rods, springs and retainers in the head.. Thats all with that set up i have seen well over 600HP on alot of cars..
THeir is no reson to sleeve your block for that kinda power. You dont need to spend 1k in head work thats crazy.. I can give u a few examples here local that there are a few cars well over 700HP and have had NO PORT AND POLISH jobs on the head. Mine for example is a stock type r head, no port and polish at all, i only upgraded the valves too 1mm and springs and retainers.. And the car is well over 700hp with that set up...

Ill give you my life expirence GSR block 100% stock i made 320hp
I later upgraded the pistons and rods and all and i was well over 550hp.
Later i wanted alot more then i got my block sleeved.