[Mike C]

Yes that sounds right. Assuming the stock springs still have the truck at stock height. There is some variance, and if your truck has settled in the last 45 years then it may lower it more than that. It may not. Be that is a good ball-park way to figure it.

On the drums, lowered coils are about the only choice. I've driven these trucks with drums, and my opinion is that they are not safe to be on the road equipped that way with newer vehicles. I consider discs to be a mandatory upgrade for a truck that will see real road duty. Not everyone agrees with me, but having done my 70 C2500 and my 69 C10 I have experience both ways with both short and long beds and both half and three quarter tons.

The beauty of the disc conversion is you can use a drop spindle and a 1 or 2 inch spring and get it as low and have much more travel.