You can get 315s in the back of that car w/o tubbing it if everything goes your way (offsets, how straight the car is, etc). 285s no problem.
IMO sit down make a plan. You'll be miles ahead. If you are going to the December meet we can talk about it.
Do a lot of homework before you start buying parts. You don't want to spend too much or buy parts that fit poorly.
Here are some Pontiac sites that are good
http://forums.performanceyears.com/forums/index.php
http://transamcountry.com/community/
This is a 2nd gen Camaro site but it may also be helpful
http://www.nastyz28.com/forum/index.php