Non syncro transmissions are much earlier, that unit is considered to be a syncromesh in spite of first gear being non syncro.
The current GO 80/90 is GL5. I believe that a GL4 or lower meets the earlier specifications. This is a subject of much discussion with many opinions as to what to use. Many use the GL5 without any problems.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine