Put the levers in the neutral position. Nothing is going to fall or spring out. Check for wear on the shift forks. The 2-3 fork is a common wear part. Also, it's generally a good idea to have a shop manual when doing this type of work. The SM318 transmission was used in 55-66 Chevy cars & trucks. You don't necessarily need a GMC manual.
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