Go back to the basics and check your work. Make sure that the shift rods are installed correctly. You may need to disconnect the shift rods to make sure that you can obtain the neutral position on both shift arms on the trans. Also check to see if the clutch is engaging correctly.
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