If you want to keep your car essentially original (which it will <span style="font-weight: bold">not</span> be with a ricer gearbox in it), go with the A-type Laycock overdrive. Properly rebuilt and maintained, the Laycock De Normanville unit will deliver immense driving pleasure for many tens of thousands of miles.
In fact, the A-type OD will reduce engine RPMs significantly more than the Toyota gearbox (5th is .83 or 17%, if I recall).
And, switching on that OD during a spirited run through the gears <span style="font-style: italic">never</span> fails to WOW passengers, many of whom have ever seen, felt or heard an overdrive unit in operation. Also, I might suggest that you first drive a Triumph with OD if you have not, I <span style="text-decoration: underline">guarantee</span> you'll love it.
The Laycock OD unit is original to the marque, it is dependable, and it's thrilling to operate: three <span style="text-decoration: underline">excellent</span> reasons, IMHO. And, did I mention that the OD is FAR more fun than the ricer 5 speed??
You'll be glad you did, if you get the OD unit.