Maybe not the most practical option, but one worth discussing to muddy the water a bit is the Borg Warner overdrive. Lloyd and probably others used to mount them in the torque tube like the Mitchell and they can be electrically operated from the driver's seat. Like the Columbia you still have some of the inherent drawbacks of an old design and probably a less rugged unit. But if you can find one or have the machinery to build your own it might be a very cost effective alternative.
And, another one that used to be very common... Volvo Laycock de Normanville. Exceptionally simple to use. Common in lower-powered old Fords (and others) Not sure if it would stand up to a real strong engine and heavy foot. In days long gone by, I've actually used both the Borg Warner, and the Volvo. Both were good at the time. Given a choice of these two, I'd probably try to find a BW. Opinion...