Jim Carter offers a rebuilt shift box. LINK
I had the 2-3 shift fork in my transmission worn a bit. I fixed it by building up the worn spot with weld and re-machined it. (PIcs below.) But your shift box could easily be your issue, if adjustment doesn't fix it.
Attachments
IMG_4422.JPG (184.24 KB, 107 downloads)
Worn 2-3 shift fork
IMG_4426.JPG (236.19 KB, 107 downloads)
Weld build-up

Kevin
1951 Chevy 3100 work truck
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Photos
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.