With your description of difficulty shifting between second and third, you're going to have to pull it and investigate. The 1st-reverse slider gear is likely not the issue. Like Dan said, it probably moved to the rear after you pulled the side cover.
[on edit] UNLESS the shifter lever in the side cover isn't working freely. Check that out first. Maybe the interlock linkage is binding up. Does the 2-3 shift drum move freely by hand in the truck? If so, the issue may be with the side cover.

Last edited by klhansen; 03/31/2026 6:19 PM. Reason: added more info
Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.