As
@clinebarger stated the 4 in the floor of that era was usually a Muncie design that required a 90'ish grade gear oil.
With much better dedicated MT gear oils today you have a choice of modern MTF's.
Those in the list below in BOLD would be my choice picks in no particular order.
MTFs in the 14-15 cSt (SAE 75W90) range would be:
1. Amsoil MTG
2. Redline MT-90
3. Castrol Syntrans Multivehicle 75W-90
4. Castrol Syntrans Transaxle 75w-90
5. Ford XT-75W90-QGT (Carries a GL-5 rating as well)
6. Ford MOTORCRAFT® Full Synthetic Manual Transmission Fluid XT-M5-QS
7. ACDelco 10-4059 GL-4 75W-90 Manual Transmission Fluid
8. RAVENOL TSG SAE 75W-90
9. LIQUIMOLY 75W-90 GL4
10. PENNZOIL 75W90 GL-4
11. MOTUL GEAR 300 GL-4/GL-5 (PRODUCT NUMBER 100118)