Maybe this will help you to identify your transmission:

Trans ID

I think I would prowl around a wrecking yard to find a gear that fits. Don't worry about the ratio at this point; just find something that will fill the hole. Next, drive the truck with a GPS to compare speedo reading with true speed. Then figure out the correction required. Probably will need an adapter.

Just an idea. I will be doing something like I suggested in the future. Or I might just permanently install a GPS.

Gord 🇨🇦
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1954 1/2 ton 235 4 speed