Yes, the driveshaft should lock in reverse with the engine off. Since "R" is the bottom of the quadrant, just adjust the linkage until the parking pawl engages and see how it affects the other quadrant positions. If there is an internal linkage problem with the reverse parking pawl that might not work.
Swapping to a manual transmission is a huge job, much more time-consuming and expensive than a Hydramatic overhaul, and butchering up an original Hydramatic truck that way should be a hanging offense!
Jerry
