I think think I see one of the dimples on a pic from the firewall of an 85 C20. It also looks like maybe a hole is there for the clutch master, but it's got a grommet in it and used for the orange wire that goes to the Th400 kickdown switch. The wire in this pic is laid up on the cowel since the motor and trans have been removed. But if you look to the left of the grommet, I think I see a dimple just outside the grommet with the single orange wire. Kinda makes me think that could be the same hole that would be used for a manual trans truck.