The exact actual length can depend on alot of things, like are you running stock springs? Lift springs or 56's in the rear? Shackle flip? Height of the truck, etc. However, I can tell you that with my 400/205 setup, I have stock 57's ford springs with the axle moved back an inch and a half, which put my axle pretty close to stock position. With that my rear d-shaft was 39 and a half inches long center yoke to center. This is on my 87 blazer.