Gearbox upgrade

[David C]

Hoping to upgrade my Torqueflite to a later box with locking torque converter and overdrive. Andrew Atherton reports excellent results from his. Sadly with Martin Barnes passing I cannot ask about the details of parts ordered for this.
I think I have identified a suitable unit but need to know if it will work with the original flex plate and whether bell housing bolt pattern is the same. Apparently gearbox extensions with manual speedo drives are available as are kits to switch overdrive and lock torque converter automatically.
Has anyone else done this who can help?