building an N/A car will not make much power. 250 at the wheels is the maximum you will make. the easiest and cheapest way to make more power would be to swap the N/A heads for supercharged heads because the injector bungs are in the heads on supercharged cars, then you could install a direct port nitrous system and make another 100 horsepower for less than 1000 dollars.
or you could save some money for the turbo kit like Sven suggested.
I may consider this
1997 Grand Prix GT 3.8(SOLD)
2002 40th Anniversary Coupe GT (SOLD)
2000 Daytona Coupe GTP (Summer Car)
2011 Ram 1500 (daily)