340 duster only good for a couple of burnouts.....

Well there you go.
Ima guessing you're blackies are pumping up the lifters and then the power goes away. I bet you have zero problems running it under 3000 all day,right?
If true,then you need better valve control;perhaps springs,or seat pressure,or anti-pump-up lifters, or less lifter preload, or shorter pushrods;or some combination of all of the above.The bottom line is control