slant six starting problems

If the ECU (spark box) was not grounded, the starter would still turn the engine over fine, but no spark = no start. Run a dedicated ground wire to a mounting screw on your ECU. Don't rely on a rusty sheet metal screw like the factory did. Buy a cheap in-line spark tester at Harbor Freight and put it in-line with a spark plug next time it won't start and see if the tester is flashing as you crank.