Recap: your are changing a torque-tube rear-end (and driveshaft/torque-tube) to an open driveshaft rear-end?
If that is correct, you are most likely making some sort of change in the spring mount, and the resultant measurements will be somewhat unique to your truck (you cannot count on this being the same length as in a 55-1st truck, unless you use a 55-1st rear-end housing)?
This is out of my small realm of experience, but, I would put the transmission and rear-end in place; then, put the u-joint assemblies in place; and then, measure the length of dive-shaft that would be needed to connect the two?
I expect appropriately rude comments about my analysis. Listen to others and ignore me.
