HEI conversion help

75slant6,
We assume you have a 1975 slant. I don't know that wiring schematic. In my cars, the thicker brown wire is only hot in "crank", termed "IGN2" to bypass the ballast. But it is not hot in "run", where "IGN" (blue?) gets powered. Since IGN isn't hot in crank, you need to connect both and run to both your ECU and coil+, so the ignition is powered in both crank and run. Coil- doesn't connect to ground. If you tried that, I bet your coil got real hot. Coil- is how the ECU controls spark (termed "low-side switching").

You have to figure out the coil wiring because you are using a purty after-market E-core coil (Ford TFI connector?). Had you used the GM 8-pin module and GM "external coil" (1985-95 V-8 pickups), no figuring since a short GM cable connects the two for easy plug n' play. That is what TrailBeast kits for those who can't go to a junkyard. I just picked all the parts a few weeks ago for a friend w/ an earlier Corvette, plus the knock sensor/module, and nice plug wires, all for ~$40 on half-off day. In his case, I grabbed the whole distributor, but we just need the 8-pin module inside it, and take the pickup connector.