the red and should go to the outside posts on the neutral safety switch i think. i am pretty sure they are for the back-up lights. you should try stripping them a bit and touching them together and see if the back-up lights come on. the other wire that you should be looking for is white i think.

 

It is an A727. It was built on March 15,1982 and is therefore not original to your truck. The black and red/black are for the back-up lights. There should be a dark green/white that comes from the starter relay for the NSS.

 

So, what does this mean for my speedometer discrepancy? Is there something obvious about the mating of a transmission form 1982 to a speedo head from 1979?

 

I am not sure when the A727 started using the reduction gear box or in what applications it is used, but I believe you will find that you need one. Take those numbers to the dealer and ask if this trans needs the reduction gear box when used in your year truck. If the trans used it in 1982 and not in 1979 you will need one. They are probably the only source to answer this question. You may also need your VIN when you ask about this.