Aftermarket Radiator

1 gauge ought to be fine for the solenoid to starter cable, I do agree with Paul (Psquare75) on using 0 gauge cable where needed, it is just a pain to work with sometimes due to the physical size and having to deal with the cable ends. The circuit for my dash lights also feeds my horn, the power for the circuit comes from the light switch of course. This is for a '79, your '75 should be set up the same way. If you can remove the light switch easily(easier than removing the dash cluster) you will need to look for a yellow wire with light blue dots, it should be on one of the four corners of the plug to the light switch. This wire supplies voltage to the printed circuit for illumination and supplies voltage for the vehicle's horn. I do not have a printed circuit for a '75 to track down the actual circuit for illumination. I hope this helps. Todd 

 Rock,
  You are absolutely correct on the light blue/red stripe wire supplying the voltage to the dash illumination that can be dimmed. I mis-quoted the wire colors, sorry for that. I had a horn wire(yellow/light blue dot)that grounded out inside the steering wheel/column and that caused the dash lights not to illuminate at all. I tore apart the dash to find the problem only to find the horn button as being the culprit. This is the reason I asked if the horn worked or not, no excuses, I should have read my wiring schematic more thoroughly before giving advice. The dash illumination is fed voltage through the light switch by a 4 amp fuse just under the T/S flasher on the left hand side of the fuse panel, it is a small fuse.