Killerjunior

I'm rebuilding a TH400 for use behind my LQ4 in a 91 camaro RS. What rebuild kit is recommended for 500+ hp? I'm not very transmission savy and I'd rather run the risk of sounding dumb than waste money on a unsatisfactory product. Also needing a flex plate for this transmission/motor combo. It's the long crank LQ4. I've seen multiple different answers and I'm confused on which route to take. This is my first build. Thanks for any input or help.