I'm a DIY guy and have removed and reinstalled them together. I found the pair had to go in almost vertically to work, and had to be careful with the the rate that I leveled the unit as it was lowered. I made sure to have a nice sliding surface where the overdrive contacted the floor. You're probably best off removing the speedometer elbow off the overdrive to avoid damage. One other consideration, you need to watch the head coming in contact with the heater box. I suggest removing the heater box or the head to avoid that problem.

Of course listening to Eric is likely your best route.