Electrical problem truck won't start

[65-72 chevytoys]

I just replaced the battery and still won't start, so I started to trace the wiring around the engine compartment. 12R from Battery to starter is good. 12B from Battery to engine block is good. I could not see the starter real well so I went the other direction. HEI Bat terminal has a 20Y that is disconnected at the starter terminal. I have been reading the FAQ section on HEI conversions and noticed that it needs a heavy guage wire that goes from the HEI bat terminal to the fuse box ignition unfused with a 30A inline fuse correct? I don't think there is a positive connection that goes from the battery to the fuse box?? Also what does the yellow wire connect to and what about the orange/purple wire from the fuse box to the starter terminal??