For a low buck compared to a crate motor alternative is get a stock older LQ4 6.0 truck motor , mild cam swap , and Holley , MSD, or Fast self tuning EFI. It will easily put you around 500hp with good drivability and hold together. I don't remember the details off the top of my head. Do a google search there are plenty of articles about it. I plan to put one of these in my dads 55 210 post with a 4L80E.
I agree with everyone else a good suspension for that kind of power is mandatory, but there are several aftermarket options available for your stock frame. For example there is a company that sells a kit to fit older (cheap) corvette front and rear suspension in trucks.
