The Doug Nash transmission is the prdecessor to the Richmond gear transmission. Doug Nash design was bought by Richmond gear when Doug Nash had financial problems around 1987. Essentially, Doug Nash was producing manual transmission planetary overdrives for the C-4 corvette and began having quality control issues. Shortly after they went belly up and Richmond gear bought them. The Richmond 6 speed is an evolution of the 5 speed.
The Doug nash 5 speed should be comparable to the richmond in terms of torque capacity and gear ratios and if you are able to get a model from about 1985, you would have a fairly stout tranny.