For future reference, the cables I posted above sold by Classic Parts and Filling Station will work with a square drive Chevy speedometer ('34-46) and a T5 if it's long enough for your application. My '36 speedometer takes a 7/8-18 screw cap, and my '85-86 S10 T5 also takes a 7/8-18 screw cap with the .104 drive pin. I called Classic Parts and confirmed the transmission end is the correct screw cap and drive pin size for a T5.
The problem may be the length, though. I might be able to just barely get it there by running it straight to the transmission, across open space, but it may be tight and could come into the transmission at a hard angle. I'd much rather run it along the frame rail and over, so I'm going to have a custom cable made from this place.
https://www.texasindustrialelectric.com/speedo.aspEDIT: Ordered from this place instead. Much cheaper.
https://www.speedoservice.com/contact/
