thats ported vacuum & my thought was that if it was manifold vacuum that it could be loosing enough(of the dist vac adv) when you cracked the throttle & that could be killing it but not the case here. Unless there's a vacuum leak I'd continue to suspect the carb but I am out of ideas . Your timing is VERY slow & I would keep it at the std SB baseline which is ~35 total(vac adv unhooked) with a reasonable initial ~10btdc or so. As long as your not pinging & it cranks Ok your timing isn't too far advanced(w your new settings) but back to original problem...