From what I've heard, if your going to add nitrous to the car and you are already at the 300HP mark N/A, then save time and troubles by purchasing a well built C4 from one of the many reputable race shops around the country. A good shop should discuss your combo, future plans, etc. Then, they can build your C4 to work with what you have and set it up with the proper stall, etc.
Good luck with the tranny!
E
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1991 5.0 LX Coupe - 38,000 miles
13.17 @ 106.14 mph w/ 2.138 60'