I would start with a good seasoned block. 350's are pretty plentiful in the boneyards. 650hp seems a lot to get real reliability and endurance from. I would go to the URLs listed earlier in the forum and start getting parts. I personaly like Summit Performance. They have good parts at reasonable prices.
I would find a machine shop that is absolutely anal and will work with you, and has experience in small block Chevys.
The biggest thing is critical machining, balance, clean work, and a willingness to not cut corners.
I like cast iron heads. I prefer used ones. I also use the machine shop to rebuild the bottom end. I like to get the crank trued to the bores, and the heads squared to the block. I do not like going more than .010 under for crank grinds. I like high volume oil pumps. I like extra capacity oil pans. I make sure of pump screen clearance. I do not use additives with detergent oil. I do like synthetic oil after 10,000 miles.
I would also ask a lot of questions, especially with manufacturers. I have found, two techs give two answers on the same question. I have also found a lot of people write a lot of things that sound good.
Good luck in your project. I hope it works.