BD - That looks like a pressed in compression plug. The bore it's in is smooth, not threaded as these later transmissions used a speedometer driven gear "plug" that was retained by a flat metal tab secured by a bolt directly behind the port opening. See screenshot below for components, not including the speedo drive/driven gears.
Can you post pictures of what you have for your speedo setup?
~ Dan 1951 Chevy 3 window 3100 Follow this story in the DITY Gallery "My Grandpa Carl's Truck and How it Became Mine" 1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver) US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)
