This is something I have been thinking about for quite some time. Would it be possible to swap between the insides of a diesel 4 speed transmission to a gasser 5 speed like one from a 190E? The exterior of both cases look the same except for the starter humps. The 5 speed gear housing looks like it just bolts on to the regular 4 speed housing. I'd go further but I don't have any FSM's on these transmissions to even attempt their disassembly.
Here are the two transmissions side by side.
This kind of idea could possibly solve the problem of getting a rare 5 speed transmission if someone is capable of doing this.