Since I haven't run a 904 under big power I can't say. Obiviously the clutch pack, bands, and sprag are up to the task. I have a friend that's run one in a 10.50 N/SS car for years. But I don't know if it has the stock output shaft. Paul Forte at Turbo Action could give you and idea on the converter threshhold limit. There are a lot of strong small blocks using 904 stuff that make good power. As far as the tiny tunnel. I'd make it big. Not that tough. Early B body has a small tunnel as well. Best thing I ever did was make it larger.
Doug
Doug
