switch to manual

I got my 65 Chevelle convertible together. It is about 375HP. The turbo 400 downshifts to 1st when I just want to downshift to 2nd, and I an nervous that I might blow it up. Also the car was a 6 cyl. car so the rear end is a 3.08 and I probably need a 3.73 or so. I'm thinking about going to...