His transmission is a lock-up style trans, not an overdrive transmission. Lockup converters do not provide any overdrive. Why not build the lockup unit and get a quality converter, and if he needs a high stall lockup they are available.
On a technicality.
Lockup converters eliminate converter slip when engaged, so technically it's not an OD you are right.
Still better economy than a standard 3 speed, and just as capable when not in lockup was my point.