the 5.0's are more my area, but in responce, i doubt your stock block will handle 500 rwhp for long, even if it can. and NO stock 3.8 makes 325 rwhp, sorry to tell ya. 500 is achievable but with alot of cash. ie-stroked, forged internals, upraded fuel system, ignition, suspension to handle it, and yes gears are a big plus =) 4.10 would get you going nicely.
__________________ 1994 Mustang GT 5.0
Mods to date: T5 swap, MGW shifter & Composite race knob, Ram clutch, flowmaster cat back system, BBK o/r h pipe, BBK equal length headers, BBK cold air intake
In the process of a gt40 install along with 4.10's and some other new goodies | 
3.8 power with boost 
