Chris, a mate of mine just did an 5.0lt EFI install about a month ago in a HZ and fitted a brand new MSD dissy/module...i dont know if the ignition module is built into the dissy or outboard like the standard EFI module....anyways i know yours isnt EFI but he fired his up and it ran fine.The next time he fired it up it backfired ran crap etc etc..had him buggered too but it all pointed to the MSD,but he didnt want to rewire everything again.
He gave up the next day and put his standard dissy/module setup and all was fine again.
There may be a bad batch of MSD stuff getting around....good to see you got it going again.
He had to return it and now is waiting for a replacement(then rewire it back again)
Sometimes the tried and true old stocky stuff just works for ever as it can be replaced asap.

cheers...mario

Edited by user Monday, 30 June 2008 1:19:23 AM(UTC)  | Reason: Not specified

Just a thought too, with the Back-firing and a Holley;
Back-fires can ruin your current power valve, if you can wind in the idle screws all the way and it still idles, your power-valve needs replacing.