D_mn Karl.
Cope - I know his rep for parts and rebuilding transmissions has been good. Didn't know his place did installs. Obviously I'd not use him for that again. Hey my friend handed back my wagon with stuff like that. Bracket on the kickdown missing - hung up the shifts first drive out, later found manifold and engine mount bolts 2-3 threads out, broke the charcoal canister's fuel tank vent connection - didn't tell me - 'fixed' it with brake line. Truth is my friend had little to do with it - his guy did the work, but that doesn't change anything. Still friendly but...they'll never work on any of my stuff again.
All my Barracuda's 727 rebuilding stuff came from A&A. Cope has a few things Rick doesn't, but I was working with Rick so that's the way I went. Don't know if they work on vehicles.
Backfire in the exhaust ought to be late timing and excess or unburned fuel.
but with 15 -20 * BTDC ???? That aint right. Spark wires crossed? - you know that bit me a couple weeks ago...
Try 10* - 20* initial with ported vac. depending on cam.
34* at idle is way too much. With an automatic, the max vac method is OK, but with the trans in Drive. With a manual, got to just go with the recommendation and play with it so works nice.