First off, the answer to a lock up converter being mandatory is no. When I purchased my 200r4 transmission it came from the builder (Art Carr, the rip-off artist) as a non-lockup OD transmission.
When I had it converted by a local guy to lockup he went through the process of trying the different styles of things to make the trans lockup automatically. Remember, the lockup style trans/converter was original equipment on vehicles that were computer controlled, and the computer would control the lockup point. Anyway, my tech settled on the adjustable style that I can set at any speed I want to for the trans to lock up, or disable it completely. Or, I can leave it off, then when I reach highway speed I can simply push the button and the trans will lockup, dropping my rpm by roughly 200.
I prefer to set it and forget it, and right now my trans locks up at around 50 mph and shifts into OD at roughly 45 mph. I can and probably should bump that up to lockup at 55 mph.