Should I be feeling more power?

Rude72* said
Mopar Performance Purple Shaft Cam P4120235

That the old 284/284 duration cam for a RB block. But you have a 340?

Rude72* said
Performance Electronic Ignition with chrome box and Accel Super Coit

There are several versions of the distributor used by MP. Timing light, timing tape and tach will be needed to find out what timing curve looks like. Search here for a how to do this.

Rude72* said
Holley 750 Carb

There's various Holley carbs with nominal 750 cfm ratings. Look at the choke tower for the "list number" or post photos. it will be 4 digits and a dash. eg 33310-4 or 4779-3

Rude72* said
When I punch it from a stop, it initially really wants to go. Once 2nd gear kicks in, the rpm/torque seem to plateau. Once in second, torque seems to diminish and it takes a few seconds (after shifting into 2nd gear) to start gaining more power.

My guesses are,
its set up too lean or possibly even too rich to pull the additional load.
timing at the rpm/throttle opening/oad is a little to low.