Have you installed the front suspension yet? if not, the need for custom headers may not be a problem with an aftermarket suspension. If that's all you can fit, I don't think it will have a problem making 1000 HP with the heads you have with 2" pipes. Perhaps 2" to 2 1/8" with a 4" merge would be an option. You know me, I only have experience with drag stuff but 1000 HP shouldn't be that difficult. You will be up in the 8500 RPM range I would think.