Weird. My truck is doing the same thing. I haven’t dug much deeper but it is a ‘62 sm420. I aslo have to shift out of 3 before stopping. It’s a PITA. I want my wife to be able to drive it and until I get that straightened out, she can’t. I switched out the 62 top hat on the transmission so I could use the hand brake pivot from the ‘50’s top hat. I read somewhere that this frequently can cause the tranny to stick in 3rd. What transmission are you using? I suspect you likely haven’t done the exact same thing as me, so I look forward to seeing what issues you may be investigating. Thanks coupe.
Fox J