Guy on DTR just did a thread about converting his a518 to lock up. Turn your non lockup 518 into a lockup on the cheap - Dodge Diesel - Diesel Truck Resource Forums

 

Chrysler did it in cars from the late 70s to the early 80s but never in the trucks, so I imagine it's entirely possible. An A518 (46rh) or A618 (47rh) is just a 727 with overdrive or overdrive/lockup, respectively, anyway. Anything after '82 or '83 and before about late '94 that was lockup had a 904 or 999, neither of which are going to handle the torque of a diesel. Your best bet is to get the hydraulic lockup transmission, adapter and passenger side drop transfer case.

I saw that lockup conversion thread. Unless you're pinched for cash or incredibly cheap, it's a lot less trouble to just source and install the 47rh.

 

I have a lockup flex plate and adapter if you need one. I'm located in the St. Louis area.