Looks good Wheelz, all legit!
Tailshaft was cast in Dec '64. They only cast a few batches a year, so that December batch lasted for a couple of months.
Stamping on the case is the assembly date. If Duane says 3-31-65, I'm not going to argue with him!
Mine's a 'WG 6 6 2, or July 6, 1965, 2nd shift. Car was built 3rd week of August.
3 groove is correct for close-ratio GS trans.
Do you see a vin stamped on the top of that mid plate/bearing support? It's between the case and the tailhousing. And is there a code painted on the side of the case in yellow? Mine is a 'KT' code, letters are about 3 to 4 inches tall.
I once bought a shifter at a swap meet for $15. It had a shorter handle, but the body was identical to the one on my 65 GS. I think the Muncie shifter is the same.