Honestly, you will not be able to make much improvement for $1500. You probably have a 250 HP motor now. Consider saving for awhile to be able to make a bigger difference.

If I was compelled to do something, it would be buy the smallest combustion chamber, low cost aftermarket aluminum head (Eddy EStreet?) and something like the Crower 271 cam. You'll probably be closer to $2000 when done, but will probably put 40+ HP in the motor. Then upgrade exhaust and induction as $ allow. You could get close to 380-390 HP and won't need to change your torque converter.