To start, I have to admit my knowledge of Ford transmissions is from the manuals, not from working on real transmissions.
It appears the gear you are looking for is part BB7101 Gear high and third sliding that was replaced by BBC 7101.
I see it listed at
https://www.lwdparts.com/product/gea...third-sliding/
BB7101 is on Mac's web site as discontinued.
It is listed as available for back-order at
https://www.oneillvintageford.co.uk/...ission_V8.html
New and used options are listed at
http://www.vanpeltsales.com/FH_web/t...ruck4speed.htm not cheap but....
A better price and closer to you
https://www.generaaltjes.nl/Adverten...da-prijs-30-00
NSN 3020-00-310-1976 seems to have many equivalent part numbers, including BB7101
BB7101 is also listed as available at
http://fliphtml5.com/pkyg/wrxl/basic where the transmission is described as a Warner T-9
I didn't find much when I searched for BBC7101