To answer the question, torsion bars can be interchanged on the '69 models they are the same length and use the same sockets, just be sure to specify that the replacements are for a Plymouth. FYI torsion bars usually will maintain a constant spring rate up until the point that they fail, they usually do not get "weak" There are several variants for the torsion bars, small block, big block police and wagon. Before doing that, check the ride height of the car, it should ride level. If you or someone else has raised the ride height, that could be the cause of the wallowing. Next thing, check the condition of the rear springs, shocks, and spring bushings, a lot of the wallowing could be from weak or damaged rear suspension components. These cars were also equipped with an optional HD front stabilizer which can also be added, that will be a donor car item. The stabilizer mounts to the lower control arms and the bottom of the front frame horns, same as the stock item. The police cruisers could be ordered with a rear stabilizer as well, be advised that installing one of those is a major PIA, you need to drill and tap frame mount holes and change out the axle mounting plate. The FSM gives detailed instructions for checking the ride height. If you have added air shocks to the rear and raised the front to get the car level, that will be the likely source of the wallowing. From you photo, the car looks to be riding about 1" too high. The other thing that happens with the ride height set too high is that the camber angle is thrown off and that messes with the steering geometry. Usually will cause poor handling and tire wear issues.
Car looks nice.
Dave