Your first mods:
1.: Make sure the car is in tip top tune up shape. Some Mobil 1, a fresh set of plugs, wires, cap, and rotor. Your car seems to be about 25 RWHP down from a stock 5.0, if it was in dire need of a tune-up, you may gain this back, this alone could make your car run around 15.20 or so at 87 MPH (Assuming a 3500lb race weight)
2: 4.10 Gear
3: Converter and shift kit. If the car is daily driven you may want to skip the converter. It doesn't make sense to spend a ton of money on a converter that is only a little looser than stock. If your intention is going fast, than get one with at least a 3000 RPM stall.
Those three will get you into the high 14's, possibly faster if your car is in sad shape right now.