Upgrading your TownAce sounds like a cool project. I remember when I wanted more out of my old truck's transmission—it really can make a difference in how smoothly it cruises at higher speeds. Swapping to a 6-speed gearbox like from a Carina GT sounds promising, though it might need some custom fitting. I once upgraded my bike's gears and saw a big improvement in efficiency.
If you're diving into research, places like Suncoast Transmission might have a 700r4 transmission for sale if you're considering options for your upgrade. Good luck with your project—I'm sure with a bit of research and maybe some help from the community.