here is a bump for you.
way back in my NU-BE days, we thought the trick to quick times was 4:88, 5:13, and 5:38 gears, no matter the tire size. well, 10.00, 11.00, and 12.00 x 15 were the tire sizes normally in use.
anyway, fast forward to the last time i had my charger out, [2006-2007, then back in storage it went] i dumped the 5:38's and installed 3:91's.
the engine was not touched other than fresh gas and new plugs.
it went within 1 tenth of the 5:38's best time.
not sure what that means, but you might want to try your 4:56's or maybe even 4:30's.
the heads and headers you mentioned couldn't hurt. i would certainly give them a try if it were me, and i don't know anything..........