The T5 is the most viable candidate, but you could also install a t56 6 speed gearbox. Back to the t5. You will need a gm bell housing found on the GM version of the 215 and early odd fire v6. They are hard to come by but there are still some out there. You will also need to install an annular sleeve type throw out bearing. Getting the correct t5 with the shifter in the right location will require some research. It's been done dozens of times on MGBV8 conversions, so I'm sure there is a comprehensive list out there. Ty asking on the British V8 website. Those guys are good at sharing info.
https://forum.britishv8.org/