Hello.
Reverse should be over to the left and down on your car. You either have a Hurst shifter or the shift lever is on upside down on the transmission. That wouldn't make the speedo run backwards, though. It could be that the speedo gear in the tail section of the tranny has been put on backwards. That would make the speedo cable turn backwards. There isn't any way to but the speedo gear on the cable on backwards. The clip that holds it wouldn't go on. Unless there's some backwards after market speed cable gear available that I'm completely unaware of, ( which is a possibility :gringreen) then you'll have to pull the tranny and flip the speedo gear on the output shaft around. Hope that helps.