Have you seen the Bull Run Chevelle that Frank at Prodigy Customs built? Really a cool car! He would be a person to ask questions.
Running naturally aspirated LS2 with heads, cam, intake and long tube headers I would think 450 at the wheels should not be too tough with good driving manners and 91-93 octane pump gas. I would also run the T-56.
I am not very familiar with what is all out there for a Chevelle, but I would click on the link to the left for the Roadster Shop. Very nice piece, full replacement frame. I have not driven one but I have seen it in action. I really do not think you could go wrong.
www.roadstershop.com
If I had the opportunity to start a car from scratch, I would get it up and running so I could drive. Sort the car out and make whatever changes and do the body and interior last.
Good luck to you!