First, clean and tighten the battery terminals. Then clean and tighten your under-hood grounds (e.g battery to block/frame, block to chassis)
If this does not cure your problem (which it probably won't, but you need clean connections anyway) check your fusible links. If they are in a similar position to the 80's trucks they will be located on the driver's fender well near the firewall. Fusible links look like normal wires. Often when they blow they do not burn up the insulation, so simply looking at them will not tell you whether they are blown. If you pull on them (grab each end and try to stretch it) and they stretch, it is blown, and is the reason you have no power.
How any of this could have happened by changing a speedo cable is beyond me. My guess is your wiring harness is full of wires with cracked insulation, and the little bit of moving around that may have been necessary to run the speedo cable could have shorted something out.... Good luck. You may want to go over your underhood and under dash wires to see if they need replacing.
-SM
Just thought of this, and just to be sure; did you unhook your ignition switch to replace the cable and forget to hook it back up?
I have a '79 D100 with a 318. I just replaced the speedo cable and now it's dead - the whole truck. Will not even crank - no lights, no wipers, no nothing. The only thing that works are the flashers. I first thought neutral safety switch, but other electrical stuff should work, right? 
