On most of my older trucks, the (truck type 3+1 floor) shifter had around a 1/2-5/8 shifter. The end had been cut down for 3/8-16 for an inch or so. This measurement was consistant from a '49 Chev 3800 (I think original) through an '80 something Ford F-100. It seems to be a standard size.
If you are making your own shifter knob, the thread can be any size convenient to you. If you are using a scrounged, or heaven forbid purchased, knob for an older truck, or a newer foreign made Xmission, the stick must have the same thread as the knob. If a floor shifter, one plausable solution would be to drill into the shifter, tap, and use a cut off bolt. I have done that once, it was more difficult than welding but didn't set the mat on fire.
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