I am in the process of back-halving an 86 vette. I have a 400 small block built right, 507hp 6500, 530 tq. on motor. Am planning on using some kind of  nitrous plate system in the future. I have a powerglide, and access to a th350. I would like to use the glide but a friend says I wont have the power and will need that third gear. I have not purchased rear gears, but have a 9inch that is already the right width. Any suggestions would be appreciated, spring is coming!
Thanks  

Depends on what youre doing with this car, it's weight and final gear ratio.

The TH350 has an additional (compared to Powerglide) lower first gear. Powerglide is essentially a 2 speed transmission that has (compared to a TH-250/350/400) second and high. The Powerglide is suitable for a competition machine where the vehicle weight is low, the rear gears stiff, and the engine power high to a point where the time it takes to shift 3 or more gears is less effective than the acceleration/speed that can be attained with 2 gears and one or more fewer shifts.

Neither a Powerglide nor the TH-350 will put up with a 500 horsepower/500 ft pounds of torque for long. Either would have to be "built" for these power levels.

Bogie  

i spent about 800 on my powerglide to update for drag racing. i run it behind a 700 hp 496 drag luv truck that wieghs 2820 with me in it.
no problems yet. i have see th350 bell housing break in half with a small block right at launch.now i'm talking about drag racing only. it would be wise to use a 400 for the street.  

I would use the glide and not the 350. i know too many people that wanted a three speed and went with a 350 instead of a 400. They all broke their 350's. A TH350 will not hold a lot of horsepower especially with nitrous. I believe you would be wasting time and money with a th350.