Here is a link to a Ford T-9 5 speed tranny for sale. Pretty sure these are the one's used on most MGB 5 speed swaps.
https://grassrootsmotorsports.com/forum/200x-classifieds/xr4ti-type-9-tranny-near-la/49571/page1/
https://grassrootsmotorsports.com/forum/200x-classifieds/xr4ti-type-9-tranny-near-la/49571/page1/