The most productive modifications are to the way the engine breathes. A good intake and exhaust system that works within the rpm and load range the engine will be operating at. A good ignition system that will help the fuel and air mix burn correctly and completely will insure you get all the engine is capable of giving. Increased compression is a time tested way to get more horsepower.
The horsepower range you are looking for is a reasonable goal for a street driven 250 if you select all the components carefully so they all work together. Santucci's book is well worth the cost when planning a 250 family engine. It covers everything from mild street engines to all out drag engines. It can be bought right here from Club Stuff.
