Some of those GM's have an anti-Knock computer (for the lack of better words). There is a pulse out of the distributer that goes to the computer and then back to the coil, not sure which wire it is, but if you check the wireing diagram you can bypass the computer, just by taking the pulse out of the distributer and connecting it directly to the coil, my friend next door ran his truck for 3 years with no knock computer just have to set your timing for the grade of gas you use most often.
