Speedometer Cable noise - what length to replace with?

[blindbug]

My Speedometer cable is obnoxiously loud. Whines like a newborn puppy when you put it in a cardboard box to sleep at night. My needle ticks 3-5mph up and down the whole time I'm driving. I need to replace the cable(s).

My question is, what kind / legth of cable do I need to get? I see in LMC Truck that there is an upper and lower cable (which I assume connect to each other in the middle). I stopped by the auto parts store and they had like (4) different length speedometer cables. It's confusing.

I have a 350 V8 out of a 1975 C20 installed in my 1986 C10, hooked up to (I believe) a 350TR. I'm assuming that the cable that fits an 86' with a 350TR will work... what is that length? Is it ok if the cable is slightly longer than what I have currently? I'm assuming that it's ok to be too long, but not ok to be too short? Also, is the cable (2) pieces (an upper / lower) or should I be looking for a single cable to run the entire length down to the transmission?

Thanks in advance for any help you can provide!