I wish I would have done more to my 340 in my 73 Challenger. Real consistent 9.75's with a stock drivetrain. I tried headers but no gain although no tuning either. 3.23 gear.
Not much you could have done, a 340 is destined to be a turd in a E or B- body.
The power to weight ratio isn't in favor of the 340. That's where the torque of a B or RB comes into play.