Was the original 1968 pos coil supply wire used? If so its a resistance wire that only runs at 8 volts and can cause headaches. If you want to try a quick fix to see if it works you can run a 12 ga. wire directly from the batt to the MSD coil. Just remember you won't be able to shut the engine off with the key and will have to pull the jumper to stop it. Don't leave it connected to the coil for very long without running the engine as it can damage the coil.

Obviously not the problem but your original 400 came with a 750 cfm quadrajet carb. I think you'll find more power with another 750 cfm carb (edelbrock, holley etc.) save the 600 for the 350 guys LOL !!!