Help on 340 combo

"And for heaven's sake, don't use torsion bars at the rear axle on a Mopar. That's why Chrysler HP suspensions were heavy duty, multi-leaf type so that torsion bars were unnecessay. The only benefit of using torsion bars on a Mopar is to crack the rear axle housing if the car locks up hard enough."

I'm sure Tom meant "traction" bars. But SS springs should last longer than 100 runs, even the "Made in Mexico" versions. My USA made leaf springs are at least 7-8 years old! Use SS springs, preferably the 002/003 should be used. OEM springs will likely not work well and probably wheel-hop, especially with soft rear shocks. I'm gonna guess you have OEM leaf springs. The harder a car leaves, the stiffer the rear shocks should be.

What stands out the most for me is that the cam is too big for the converter and shift points. The single plane Victor intake is also too big.

I don't use a trans brake so I'm not sure of the carb set-up, but the brake can make it more difficult to get the suspension right to launch it.

Even though you have 9" wide slicks, 26" tall is pretty short, especially with a 4.56 gear. Go as tall as you have clearance for. But don't worry about more gear until you get a handle on the launches. But I doubt it will work at all even with SS springs & good shocks if you try to launch with NOS.

Power falling off up top can be one of a lot of things, including valve float.
Are there power valves in the carb?

The basic engine combo sounds ok, as long as cam is installed correctly, valves adjusted properly, and other such details.