ok - might have to sit down and read for a bit...
You have a lot of options for your motor and tranny. Any motor can be built to make power if the right parts are choosen, money spent wisely, and the builder is knowledgible of the motor he is building.
You have money in your car already right? You don't want to loose money on your car right? (we all don't). Stay with Pontiac motors for this reason. Putting a chevy motor in your car kills the value and if you every wanted or had to sell your car it would be harder to sell and you would get less for the car.
I understand getting a turn key motor is very desirable. You can save some money some times and you save a lot of time. (pop it in and go theory).
Check out PDUDE's (Ken Keefer). He offers some Pontiac "crate motors" which are a great deal.
http://www.angelfire.com/fl4/pontiac...chineshop.html or
http://www.pontiacdude.cc He has a "12 second combo" for like $5,000. Pump gas and can drive it every day. He's been into F-Bodies for a long time and will be able to get a motor that will give the power in your car that you are looking for.
Understand this
500 hp's on the road is a lot to handle. I was into all the hype of "needing" huge horse to be happy. But if you ever been in a car with 400 horse... it can blow away most things on the street (with the coorect suspension, gear ratio, and tires). ... one of my cars (68 GTO) with 405 hps is a blast. I put basic BF Goodrich TA's on it one day. I could barely take off with our spinning the tires (remember Pontiac motors are supper torquey!) and when I jumped on the gas and shifted the four speed I laid rubber in a four gears ... and we're talking long patches in all gears! It was crasy...
TH350 is a little smaller, lighter, and takes less HPs to run then a TH400 and will more more then fine for 400-500 range. An engine builder can hep select a torque converter to match up with your gears ans cam /etc to make your car work together - thus making it fast.
Another option I would think about is the Pontiac 350 stroker "Pontiac 383". High Performance Pontiac had a big righ up about these motors that made killer horse for a little less money then our beloved 400s. ACE Brewer
http://www.pacificperformanceracing.com sells the rotation assemblies but any of the big engine builders can get them and build what you want.
2-cents