While you have both side covers off look at the 2-3 shift fork. It is a common wear point and can cause shifting and popping out of gear problems. Use the best one or try to locate a new one.
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