First off I would start with a 400 block if possible. The extra bore diameter makes things easier. I'd also limit the engine to a 4.250 stroke and use a 2.200 pin size with a Chevy rod. Probably 6.70 long but that can vary depending on things. Compression ratio should 10.0 or less to run on 91. Maybe even put it in the low to mid 9's. Use Trick Flow 240 heads and a modest hyd roller cam and you'll be good to go. Might make 600 hp if everything is done just right but I wouldn't bet on it. Should be in the 550 range without any trouble though.