ATmega328P running on 3.3V using a CH340G as serial to USB interface
I'm surprised you found a datasheet on CH430G. There are no US distributors of the chip, I won't use it in my designs.  FT232 has an IO voltage pin where the IO level can be selected.


  wouldn't one need additional resistors on the D+ and D- to drop the USB voltage of 5V to 3.3V on these data lines as well?


I'm pretty sure the D+/D- lines are not 0/5V but +/-3.3V similar to RS485. Half duplex communication between PC and the device.

The D+/D- mines often have like a 22 or 33 ohm series resistor for termination reasons (internal on the FT232) and also transzorbs from signal to Gnd to prevent ESD damage.


  CH340 chip have built in USB bus pull-up resistors and on-chip signal termination, UD+ and UD- pins should be connected to the USB bus lines directly. 


So, no series resistor needed.


  2 TXD Output UART Data Transmit output.
  3 RXD Input UART Data Receive input.


So 2 goes to uC Rx input, 3 goes to uC Tx output.  If your uC is 3.3V powered, then power the chip from 3.3V as well.