Get 4.10 gears or 4.30's if its a 340 engine, that is whats holding it back. Lock the timing out to 32-34 degrees--32 might be best, too much timing
can hurt et's
Single plane intake, a 850 DP and 4.56 gears will get the best et's out of a 340. Headers the bigger the better. The carb might even et best with those domantars, 1000 cfm's